Display device

ABSTRACT

A electroluminescence display device includes a pixel including a selection transistor, a driving transistor, and an EL element, a scanning signal line electrically connected with a gate of the selection transistor, a data signal line electrically connected with a source of the selection transistor, and a carrier injection amount control signal line applying a voltage to the EL element. The EL element includes a first electrode, a third electrode, a first insulating layer between the first electrode and the third electrode, an electron transfer layer between the first insulating layer and the third electrode, a light emitting layer containing an electroluminescence material between the electron transfer layer and the third electrode, and a second electrode located outer to a region where the first electrode, the first insulating layer, the electron transfer layer and the third electrode overlap each other, the second electrode being in contact with the electron transfer layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/678,229 filed on Nov. 8, 2019, which claims the benefit of priorityfrom the prior Japanese Patent Application No. 2019-029934 filed on Feb.22, 2019, the entire contents of which are incorporated herein byreference.

FIELD

An embodiment of the present invention relates to a structure of, and adriving system for, a display device in which a pixel includes anelectroluminescence element (hereinafter, referred to also as an “ELelement”).

BACKGROUND

An organic electroluminescence element (organic EL element) includes apair of electrodes referred to as a “positive electrode” and a “negativeelectrode”) and a light emitting layer located between the pair ofelectrodes. A basic structure of an organic EL element includes twoterminals. However, a three-terminal organic EL element including athird electrode in addition to the two terminals has been disclosed.

For example, an organic EL element including a positive electrode, alayer formed of an organic electroluminescence material and referred toas a “light emitting material layer”, a negative electrode, and anauxiliary electrode facing the negative electrode and the light emittingmaterial layer with an insulating layer being provided between theauxiliary electrode and the negative electrode/the light emittingmaterial layer is disclosed (see Japanese Laid-Open Patent PublicationNo. 2002-343578). A light emitting transistor including a hole injectionlayer, a carrier dispersion layer, a hole transfer layer and a lightemitting layer that are stacked in this order, between a positiveelectrode and a negative electrode, from the side of the positiveelectrode, and further including an auxiliary electrode facing thepositive electrode with an insulating film being provided between theauxiliary electrode and the positive electrode has been disclosed (seeWO2007/043697).

An organic light emitting transistor element including an auxiliaryelectrode, an insulating film provided on the auxiliary electrode, afirst electrode provided with a predetermined size on the insulatingfilm, a charge injection suppression layer provided on the firstelectrode, a charge injection layer provided on a region of theinsulating film where the first electrode is not provided, a lightemitting layer provided on the charge injection suppression layer andthe charge injection layer or provided on the charge injection layer,and a second electrode provided on the light emitting layer has beendisclosed (see Japanese Laid-Open Patent Publication No. 2007-149922 andJapanese Laid-Open Patent Publication No. 2007-157871).

A known example of device in which the organic EL element is usable is adisplay device. An organic electroluminescence display device includes adisplay, in which a plurality of pixels is arrayed. Each of the pixelsincludes an organic EL element, a driving transistor driving the organicEL element, a selection transistor to which a scanning signal is to beinput, and the like. The driving transistor and the selection transistorare each formed of a thin film transistor by use of an amorphous siliconsemiconductor, a polycrystalline silicon semiconductor or an oxidesemiconductor (see Japanese Laid-Open Patent Publication No. 2007-053286and Japanese Laid-Open Patent Publication No. 2014-154382). A displaydevice including an electroluminescence element (hereinafter, referredto as an “EL element”) is practically used in a multi-functional mobilephone called a “smartphone”. Practical application of a display deviceincluding an EL element to a TV is now in progress. In order to befurther spread, the display device including an EL element is desired tobe improved in the reliability.

SUMMARY

A electroluminescence display device in an embodiment according to thepresent invention includes a pixel including a selection transistor, adriving transistor, and an EL element, a scanning signal lineelectrically connected with a gate of the selection transistor, a datasignal line electrically connected with a source of the selectiontransistor, and a carrier injection amount control signal line applyinga voltage to the EL element. The EL element includes a first electrode,a third electrode including a region facing the first electrode, a firstinsulating layer between the first electrode and the third electrode, anelectron transfer layer between the first insulating layer and the thirdelectrode, a light emitting layer containing an electroluminescencematerial between the electron transfer layer and the third electrode,and a second electrode located outer to a region where the firstelectrode, the first insulating layer, the electron transfer layer andthe third electrode overlap each other, the second electrode being incontact with the electron transfer layer. The first electrode iselectrically connected with the carrier injection amount control signalline, the second electrode is connected with a drain of the drivingtransistor, and the third electrode is supplied with a constant voltage.

A electroluminescence display device in an embodiment according to thepresent invention includes a plurality of pixels each including aselection transistor, a driving transistor, and an EL element, theplurality of pixels being arrayed in a first direction and in a seconddirection crossing the first direction, a plurality of scanning signallines electrically connected with gates of the selection transistors,the plurality of scanning signal lines extending in the first directionand being arrayed in the second direction, a plurality of data signallines electrically connected with sources of the selection transistors,the plurality of data signal lines extending in the second direction andbeing arrayed in the first direction, and a plurality of carrierinjection amount control signal lines connected with the EL elements,the plurality of carrier injection amount control signal lines extendingin the first direction or the second direction and being arrayed in thesecond direction or the first direction. The EL element of each of theplurality of pixels includes a first electrode, a third electrodeincluding a region facing the first electrode, a first insulating layerbetween the first electrode and the third electrode, an electrontransfer layer between the first insulating layer and the thirdelectrode, a light emitting layer containing an electroluminescencematerial between the electron transfer layer and the third electrode,and a second electrode located outer to a region where the firstelectrode, the first insulating layer, the electron transfer layer andthe third electrode overlap each other, the second electrode being incontact with the electron transfer layer. The first electrode iselectrically connected with one of the carrier injection amount controlsignal lines, the second electrode is connected with a drain of thedriving transistor, and the third electrode is supplied with a constantvoltage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional structure of an organic EL element usablein an EL display device according to an embodiment of the presentinvention;

FIG. 2 shows a cross-sectional structure of an organic EL element usablein the EL display device according to an embodiment of the presentinvention;

FIG. 3 shows an operation state of an EL element usable in the ELdisplay device according to an embodiment of the present invention whilea carrier injection amount control electrode is floating;

FIG. 4 shows an operation state of an EL element usable in the ELdisplay device according to an embodiment of the present invention whilethe carrier injection amount control electrode is supplied with a lowvoltage;

FIG. 5 shows an operation state of an EL element usable in the ELdisplay device according to an embodiment of the present invention whilethe carrier injection amount control electrode is supplied with a highvoltage;

FIG. 6 shows an energy band structure of an EL element usable in the ELdisplay device according to an embodiment of the present invention;

FIG. 7 shows an energy band structure of a light emitting layer, anelectron blocking layer and a hole blocking layer of the EL element;

FIG. 8 schematically shows a current vs. voltage characteristic of an ELelement usable in the EL display device according to an embodiment ofthe present invention;

FIG. 9 shows a structure of the EL display device according to anembodiment of the present invention;

FIG. 10 shows an equivalent circuit of a pixel of the EL display deviceaccording to an embodiment of the present invention;

FIG. 11 shows an equivalent circuit of a pixel of the EL display deviceaccording to an embodiment of the present invention;

FIG. 12 shows waveforms and timings of signals applied to a scanningsignal line and a carrier injection amount control signal line of the ELdisplay device according to an embodiment of the present invention;

FIG. 13A shows an example of waveform showing that the voltage of asignal applied to the carrier injection amount control signal line ofthe EL display device according to an embodiment of the presentinvention increases in a step-like manner;

FIG. 13B shows an example of waveform showing that the voltage of asignal applied to the carrier injection amount control signal line ofthe EL display device according to an embodiment of the presentinvention has a period in which the voltage is kept at a constant levelduring a period in which the voltage decreases continuously;

FIG. 14A shows an example of waveform showing that the voltage of asignal applied to the carrier injection amount control signal line ofthe EL display device according to an embodiment of the presentinvention increases and then decreases in a sine wave manner;

FIG. 14B shows an example of waveform showing that the voltage of asignal applied to the carrier injection amount control signal line ofthe EL display device according to an embodiment of the presentinvention decreases and then increases in a sine wave manner;

FIG. 15A shows an example of waveform showing that the voltage of asignal applied to the carrier injection amount control signal line ofthe EL display device according to an embodiment of the presentinvention decreases in a step-like manner a plurality of times in oneframe period;

FIG. 15B shows an example of waveform showing that the voltage of asignal applied to the carrier injection amount control signal line ofthe EL display device according to an embodiment of the presentinvention increases in a step-like manner a plurality of times in oneframe period;

FIG. 16A shows an example of waveform showing that the voltage of asignal applied to the carrier injection amount control signal line ofthe EL display device according to an embodiment of the presentinvention increases and then decreases in a sine wave manner a pluralityof times in one frame period;

FIG. 16B shows an example of waveform showing that the voltage of asignal applied to the carrier injection amount control signal line ofthe EL display device according to an embodiment of the presentinvention decreases and then increases in a sine wave manner a pluralityof times in one frame period;

FIG. 17 shows a circuit configuration of a display of the EL displaydevice according to an embodiment of the present invention;

FIG. 18 shows waveforms of signals applied to the scanning signal lineand the carrier injection amount control signal line of the EL displaydevice according to an embodiment of the present invention;

FIG. 19 shows a circuit configuration of a display of the EL displaydevice according to an embodiment of the present invention;

FIG. 20 shows waveforms of signals applied to the scanning signal lineand the carrier injection amount control signal line of the EL displaydevice according to an embodiment of the present invention;

FIG. 21 schematically shows a method for driving the EL display device;

FIG. 22 shows waveforms of signals applied to the scanning signal lineand the carrier injection amount control signal line of the EL displaydevice according to an embodiment of the present invention;

FIG. 23 shows a circuit configuration of a display of the EL displaydevice according to an embodiment of the present invention;

FIG. 24 shows a circuit configuration of a display of the EL displaydevice according to an embodiment of the present invention;

FIG. 25 shows a planar structure of a pixel of the EL display deviceaccording to an embodiment of the present invention;

FIG. 26A shows a cross-sectional structure of the pixel of the ELdisplay device according to an embodiment of the present invention,taken along line A1-A2 in FIG. 25;

FIG. 26B shows a cross-sectional structure of the pixel of the ELdisplay device according to an embodiment of the present invention,taken along line B1-B2 in FIG. 25;

FIG. 27 shows a planar structure of a pixel of the EL display deviceaccording to an embodiment of the present invention;

FIG. 28A shows a cross-sectional structure of the pixel of the ELdisplay device according to an embodiment of the present invention,taken along line A3-A4 in FIG. 27;

FIG. 28B shows a cross-sectional structure of the pixel of the ELdisplay device according to an embodiment of the present invention,taken along line B3-B4 in FIG. 27;

FIG. 29 is a plan view showing a partial structure of pixels in the n'throw and the (n+1)'th row of the EL display device according to anembodiment of the present invention;

FIG. 30A shows a cross-sectional structure of the EL display deviceaccording to an embodiment of the present invention, specifically, astructure of a connection portion of a first data signal line and aselection transistor of a pixel in the n'th row;

FIG. 30B shows a cross-sectional structure of the EL display deviceaccording to an embodiment of the present invention, specifically, astructure of a connection portion of a second data signal line and aselection transistor of a pixel in the (n+1)'th row;

FIG. 31A shows a cross-sectional structure of the EL display deviceaccording to an embodiment of the present invention, specifically, astructure of the connection portion of the first data signal line andthe selection transistor of the pixel in the n'th row; and

FIG. 31B shows a cross-sectional structure of the EL display deviceaccording to an embodiment of the present invention, specifically, astructure of the connection portion of the second data signal line andthe selection transistor of the pixel in the (n+1)'th row.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings and the like. The present invention may becarried out in various embodiments, and should not be construed as beinglimited to any of the following embodiments. In the drawings, componentsmay be shown schematically regarding the width, thickness, shape and thelike, instead of being shown in accordance with the actual sizes, forthe sake of clear illustration. The drawings are merely examples and donot limit the present invention in any way. In the specification and thedrawings, components that are substantially the same as those describedor shown previously bear the identical reference signs thereto (or theidentical reference signs followed by letters “a”, “b” or the like), anddetailed descriptions thereof may be omitted. The terms “first”,“second” and the like used for elements are merely provided fordistinguishing the elements and do not have any other significanceunless otherwise specified.

In the specification and the claims, an expression that a component is“on” another component encompasses a case where such a component is incontact with another component and also a case where such a component isabove or below the another component, namely, a case where still anothercomponent is provided between such a component and the anothercomponent, unless otherwise specified.

1. Organic EL Element

A structure and an operation of an EL element according to an embodimentof the present invention will be described.

1-1. Structure of the EL Element

An EL element has a structure in which a positive electrode (or anegative electrode), an organic layer and a negative electrode (or apositive electrode) are stacked on a substrate. The organic layer has astructure in which a light emitting layer containing an EL material andalso layers referred to by names representing functions thereof, such asa hole injection layer, a hole transfer layer, an electron injectionlayer, an electron transfer layer, a hole blocking layer and an electronblocking layer, are stacked in an appropriate manner. One or a pluralityof these layers referred to by the names representing the functionsthereof may be omitted from the organic layer. In order to allow the ELelement to emit light, the negative electrode acts as an electrode thatinjects electrons into the organic layer, whereas the positive electrodeacts as an electrode that injects holes into the organic layer.

The EL element has a structure in which light emitted by the lightemitting layer is output through one of the positive electrode and thenegative electrode. The EL element is classified into one of abottom-emission type and a top-emission type based on the direction inwhich the light is output. In the bottom-emission type, the light isoutput through the substrate. In the top-emission type, the light isoutput from the side opposite to the substrate. The EL element may beclassified based on the order in which the electrodes and the organiclayers are stacked during the production thereof. For example, the ELelement is classified into one of a so-called forward stack structureand a reverse stack structure. In the forward stack structure, thepositive electrode, the hole transfer layer, the light emitting layer,the electron transfer layer and the negative electrode are stacked onthe substrate in this order. In the reverse stack structure, the layersare stacked in an opposite order thereto.

1-1-1. Bottom Emission-Type Organic EL Element

FIG. 1 shows an example of cross-sectional structure of an organic ELelement 102 a. The organic EL element 102 a shown in FIG. 1 has astructure in which a first electrode 124, a first insulating layer 126,a second electrode 128, an electron transfer layer 130 (a first electrontransfer layer 130 a and a second electron transfer layer 130 b), anelectron injection layer 132, a light emitting layer 134, a holetransfer layer 136, a hole injection layer 138, and a third electrode140 are stacked on a substrate 110 in this order from the side of thesubstrate 110. The second electrode 128 corresponds to the negativeelectrode, and is provided to be in contact with the electron transferlayer 130 (first electron transfer layer 130 a). The third electrode 140corresponds to the positive electrode.

Although not shown in FIG. 1, a hole blocking layer may be locatedbetween the electron injection layer 132 and the light emitting layer134, and an electron blocking layer may be located between the lightemitting layer 134 and the hole transfer layer 136. Among the organiclayers, one of the hole transfer layer 136 and the hole injection layer138 may be omitted, or the hole transfer layer 136 and the holeinjection layer 138 may be replaced with a hole injection and transferlayer having a function of hole injection and a function of holetransfer.

The organic EL element 102 a includes a region (overlapping region)where the first electrode 124, the first insulating layer 126, theelectron transfer layer 130 (the first electron transfer layer 130 a andthe second electron transfer layer 130 b), the electron injection layer132, the light emitting layer 134, the hole transfer layer 136, the holeinjection layer 138 and the third electrode 140 overlap each other in avertical direction (thickness direction). This overlapping region is alight emitting region 148 of the EL element 102 a. Since the EL element102 a is of a bottom-emission type, the light output from the lightemitting layer 134 is output through the first electrode 124. Therefore,the first electrode 124 is formed of a conductive material that islight-transmissive. By contrast, the third electrode 140 is formed of ametal film or a stack structure of a transparent conductive film and ametal film, so as to reflect the light output from the light emittinglayer 134.

The second electrode 128 is located outer to the light emitting region148, and is located to be electrically connected with the electrontransfer layer 130 (first electron transfer layer 130 a). The secondelectrode 128 is provided to be in contact with at least a part of theelectron transfer layer 130 (first electron transfer layer 130 a). Forexample, the second electrode 128 is located between the electrontransfer layer 130 (first electron transfer layer 130 a) and the firstinsulating layer 126, such that a top surface of the second electrode128 is in contact with a bottom surface of the electron transfer layer130 (first electron transfer layer 130 a).

FIG. 1 shows an embodiment in which the second electrode 128 includes ametal oxide conductive layer 128 a, which is conductive, and a metallayer 128 b stacked on each other. Since the metal oxide conductivelayer 128 a is in contact with the electron transfer layer 130 (firstelectron transfer layer 130 a), the second electrode 128 decreases thecontact resistance thereof. The metal layer 128 b is provided as anauxiliary electrode decreasing the sheet resistance (planar resistivity)of the metal oxide conductive layer 128 a. The metal layer 128 bprovided in contact with the metal oxide conductive layer 128 acontributes to decreasing the resistance of the second electrode 128 andthus contributes to decreasing the serial resistance component of the ELelement 102 a.

In the EL element 102 a, the second electrode 128 is an electrode thathas a function of injecting electrons into the electron transfer layer130 and is referred to as a “negative electrode” or a “cathode”. Bycontrast, the third electrode 140 is an electrode that has a function ofinjecting holes into the hole injection layer 138 and is referred to asa “positive electrode” or an “anode”. The electrons are injected fromthe second electrode 128 into the electron transfer layer 130, and theholes are injected from the third electrode 140 into the hole transferlayer 138, and as a result, the EL element 102 a emits light. The firstelectrode 124 is insulated from the electron transfer layer 130 and thusis not an electrode that directly injects carriers. However, the firstelectrode 124 is located such that an electric field generated byapplication of a voltage to the first electrode 124 acts on the electrontransfer layer 130. The electric field formed by the first electrode 124influences the distribution of the carriers in the electron transferlayer 130, and also has a function of controlling the amount of thecarriers to flow into the light emitting layer 134. Because of suchfunctions, the first electrode 124 is referred to also as a “carrierinjection amount control electrode”, and is used as an electrode thatcontrols the light emission state of the EL element 102 a.

FIG. 1 shows an embodiment in which the electron transfer layer 130includes two layers, more specifically, the first electron transferlayer 130 a and the second electron transfer layer 130 b. The firstelectron transfer layer 130 a is located to be in contact with thesecond electrode 128, whereas the second electron transfer layer 130 bis located closer to the light emitting layer 134 than the firstelectron transfer layer 130 a. The first electron transfer layer 130 aand the second electron transfer layer 130 b are common in the functionof transferring the carriers (electrons) injected from the secondelectrode 128 to the light emitting layer 134. The first electrontransfer layer 130 a in contact with the second electrode 128 and thesecond electron transfer layer 130 b located closer to the lightemitting layer 134 than the first electron transfer layer 130 a aredifferent from each other in the carrier (electron) concentration andthe carrier (electron) mobility. Specifically, the carrier (electron)concentration of the second electron transfer layer 130 b has a valuelower than that of the carrier (electron) concentration of the firstelectron transfer layer 130 a in order to prevent the deactivation ofexcitons.

The second electron transfer layer 130 b may be omitted from the ELelement 102 a. Namely, the first electron transfer layer 130 a may be incontact with the electron injection layer 132. The first electrontransfer layer 130 a and the second electron transfer layer 130 b havethe common function of transferring the electrons injected from thesecond electrode 128 to the light emitting layer 134, and therefore, maybe functionally regarded as being one layer.

The EL element 102 a may include an insulating layer having an openingin order to demarcate the light emitting region 148. FIG. 1 shows anembodiment in which a second insulating layer 142 having a first opening146 a is provided between the first electron transfer layer 130 a andthe second electron transfer layer 130 b. The second insulating layer142 is provided as an upper layer to the first electron transfer layer130 a (provided on the side opposite to the substrate 110). The secondinsulating layer 142 has the first opening 146 a exposing a part of atop surface of the first electron transfer layer 130 a. The secondelectron transfer layer 130 b is provided as an upper layer to thesecond insulating layer 142 (provided on the side opposite to thesubstrate 110) to be in contact with the first electron transfer layer130 a in the first opening 146 a.

In the first opening 146 a, the first electron transfer layer 130 a, thesecond electron transfer layer 130 b, the electron injection layer 132,the light emitting layer 134, the hole transfer layer 136, the holeinjection layer 138 and the third electrode 140 are stacked in series soas not to inhibit the flow of the carriers (electrons, holes). Theregion where these layers are stacked in series is the light emittingregion 148. Namely, the light emitting region 148 of the EL element 102a is demarcated by the first opening 146 a.

FIG. 1 shows an embodiment in which a third insulating layer 144 isprovided as an upper layer to the second insulating layer 142 (providedon the side opposite to the substrate 110). In terms of the stackingorder, the third insulating layer 144 is provided between the secondinsulating layer 142 and the second electron transfer layer 130 b. Thethird insulating layer 144 has a second opening 146 b overlapping thefirst opening 146 a. The third insulating layer 144 has the secondopening 146 b to demarcate the light emitting region 148. The secondelectrode 128 is covered with the second insulating layer 142 to beinsulated from the third electrode 140. The third insulating layer 144provided on the second insulating layer 142 extends the distance betweenthe second electrode 128 and the third electrode 140. Such a structuredecreases the parasitic capacitance formed in a region where the secondinsulating layer 142 and the third electrode 140 overlap each other. Thethird insulating layer 144 may be omitted from the EL element 102 a.

It is preferred that the second opening 146 b of the third insulatinglayer 144 has a diameter that is equal to, or longer than, that of thefirst opening 146 a of the second insulating layer 142. With thestructure in which the diameter of the second opening 146 b is longerthan the diameter of the first opening 146 a, steps are formed. It ispreferred that the first opening 146 a of the second insulating layer142 and the second opening 146 b of the third insulating layer 144 eachhave a side surface inclining so as to open upward (the side opposite tothe substrate 110). In the case where the side surfaces of the firstopening 146 a and the second opening 146 b are inclining, the steepnessof the steps is alleviated. Such a structure prevents the steps of thefirst opening 146 a and the second opening 146 b from causing cracks inthe second electron transfer layer 130 b, the electron injection layer132, the light emitting layer 134, the hole transfer layer 136, the holeinjection layer 138 or the third electrode 140.

The second electrode 128 is located outer to the first opening 146 a ofthe second insulating layer 142 so as not to be exposed to the firstopening 146 a. As shown in FIG. 1, an end of the second electrode 128 islocated away from an opening end of the second insulating layer 142. Inother words, the second electrode 128 is located away from the lightemitting region 148. The end of the second electrode 128 and an end ofthe light emitting region 148 are connected with each other via thefirst electron transfer layer 130 a. A region between the end of thesecond electrode 128 and the end of the light emitting region 148 isreferred to as an “offset region 150”. The offset region 150 is a regionwhere the first electron transfer layer 130 a is held between the firstinsulating layer 126 and the second insulating layer 142, and overlapsthe first electrode 124. Where a total thickness of the electrontransfer layer 130, the electron injection layer 132, the light emittinglayer 134, the hole transfer layer 136 and the hole injection layer 138is 100 nm to 1000 nm, it is preferred that the offset region 150 has alength that is longer than, or equal to, ten times the total thickness,specifically, a length of about 1 μm to about 20 μm, for example, 2 μmto 5 μm, for the purpose of preventing the concentration of the electricfield. The EL element 102 a includes the offset region 150 having such astructure to have an increased withstand voltage.

The amount of the carriers (electrons) to be transferred from theelectron transfer layer 130 (the first electron transfer layer 130 a andthe second electron transfer layer 130 b) to the light emitting layer134 is controlled by the first electrode 124. As the voltage applied tothe first electrode 124 is increased, the electric field acting on theelectron transfer layer 130 (the first electron transfer layer 130 a andthe second electron transfer layer 130 b) is increased. The electricfield generated by an application of a positive voltage to the firstelectrode 124 acts to draw the carriers (electrons) from the secondelectrode 128 to the first electron transfer layer 130 a. Therefore, theamount of the carriers (electrons) transferred to the light emittinglayer 134 may be increased. Namely, the amount of the carriers(electrons) transferred from the first electron transfer layer 130 a tothe light emitting layer 134 may be controlled by the level of thevoltage applied to the first electrode 124. The first electrode 124 maydirectly control the amount of the carriers (electrons) to be injectedinto the light emitting layer 134. This property is usable to balancethe amount of the carriers (electrons) injected into the light emittinglayer 134 from the first electron transfer layer 130 a and the amount ofthe carriers (holes) injected into the light emitting layer 134 from thethird electrode 140 (this balance between the electrons and the holes isreferred to as the “carrier balance”). Namely, the EL element 102 a hasan element structure capable of adjusting the carrier balance.

The electric field formed by the first electrode 124 also acts on theoffset region 150. When a positive voltage is applied to the firstelectrode 124, the carriers (electrons) are induced to a part of thefirst electron transfer layer 130 a that is in the offset region 150.Such an action prevents the offset region 150 from having an increasedresistance. In the case where the length of the offset region 150 isabout 1 μm to about 5 μm, when the first electrode 124 has the groundpotential, the electrons are prevented from flowing from the secondelectrode 128 to the first electron transfer layer 130 a.

The electron transfer layer 130 (the first electron transfer layer 130 aand the second electron transfer layer 130 b) is formed of alight-transmissive oxide semiconductor. The light-transmissive oxidesemiconductor is an inorganic material and an oxide, and therefore, hasa thermal stability higher than that of an organic material. Since theelectron transfer layer 130 is formed of an oxide semiconductor, the ELelement 102 a may have a stable structure with no deterioration in thecharacteristics even though having a reverse stack structure.

1-1-2. Top-Emission EL Element

FIG. 2 shows an EL element 102 b of the top-emission type. The ELelement 102 b of the top-emission type has a structure same as that ofthe EL element 102 a of the bottom-emission type shown in FIG. 1 exceptfor the structures of the third electrode 140 and the first electrode124. In the case of the EL element 102 b of the top-emission type, thefirst electrode 124 is formed of a metal film such that a lightreflective surface is formed, and the third electrode 140 is formed of atransparent conductive film such that the light output from the lightemitting layer 134 is transmitted through the third electrode 140. Thesecond electrode 128 is located outer to the light emitting region 148,and therefore, has the same structure as that of the second electrode128 of the EL element 102 a of the bottom-emission type. The organiclayers have a structure in which the electron transfer layer 130 (thefirst electron transfer layer 130 a and the second electron transferlayer 130 b), the electron injection layer 132, the light emitting layer134, the hole transfer layer 136 and the hole injection layer 138 arestacked on the substrate 110 in this order from the side of thesubstrate 110.

As described above, the EL element 102 b of the top-emission type isrealized by changing the structures of the first electrode 124 and thethird electrode 140. Namely, an EL element 102 according to thisembodiment may be provided either as a bottom emission-type element or atop emission-type element by changing the structures of the electrodeswhile keeping the reverse stack structure.

1-2. Operation of the EL Element

With reference to FIG. 3, FIG. 4 and FIG. 5, an operation of the ELelement 102 will be described. FIG. 3, FIG. 4 and FIG. 5 eachschematically show the positional arrangement of the first electrode124, the first insulating layer 126, the second electrode 128, theelectron transfer layer 130, the electron injection layer 132, the lightemitting layer 134, the hole transfer layer 136, the hole injectionlayer 138 and the third electrode 140, which are included in the ELelement 102.

1-2-1. Operation of Light Emission and Light Non-Emission of the ELElement

The EL element 102 emits light by a forward current flowing both in thethird electrode (positive electrode) 140 and the second electrode(negative electrode) 128. FIG. 3 shows an embodiment in which the firstelectrode (carrier injection amount control electrode) 124 is connectedwith a carrier injection amount control signal line Gn, the secondelectrode 128 is connected with a drain of a driving transistor 106, andthe third electrode 140 is connected with a power supply line PL.

FIG. 3 shows a state where a voltage (Vdata) based on a data signal isapplied to a gate of the driving transistor 106, the EL element 102 isbiased forward (biased such that the third electrode 140 has a potentialhigher than that of the second electrode 128), and the first electrode124 is floating. When the forward voltage applied to the EL element 102is higher than, or equal to, the light emission start voltage of the ELelement 102, holes are injected from the third electrode, whereaselectrons are injected from the second electrode 128. The light emissionintensity of the EL element 102 is controlled by the magnitude of theforward current. The current flowing in the EL element 102 is controlledby a drain current of the driving transistor 106. The drain current ofthe driving transistor 106 is controlled by the gate voltage (Vdata)held by a capacitive element 108.

The EL element 102 has a structure in which the electron transfer layer130 (the first electron transfer layer 130 a and the second electrontransfer layer 130 b) and the third electrode 140 face each other whilehaving the light emitting layer 134 therebetween on the first insulatinglayer 126, and the second electrode 128 does not overlap the thirdelectrode 140 but is connected with the third electrode 140 in aperipheral region of the first electron transfer layer 130 a. In the ELelement 102, the electrons are injected from the second electrode 128into the first electron transfer layer 130 a, whereas the holes areinjected from the third electrode 140 into the hole injection layer 138.Since the first electrode 124 has a floating potential, the electronsinjected into the first electron transfer layer 130 a is drifted only bythe electric field generated between the third electrode 140 and thesecond electrode 128. In this case, the electric field distributiongenerated between the second electrode 128 and the third electrode 140is not uniform in the light emitting region 148 due to the structure ofthe EL element 102.

In this state, even if a forward voltage higher than, or equal to, thelight emission start voltage is applied to the EL element 102, thecarriers (electrons) injected from the second electrode 128 into thefirst electron transfer layer 130 a do not flow in the light emittingregion 148 with a uniform concentration distribution. Therefore, acentral region of the light emitting region 148 of the EL element 102 isdark, whereas a peripheral region of the light emitting region 148 emitsbright light. In addition, the amount of the current flowing in the ELelement 102 is small and the light emission intensity is not high due tothe influence of the offset region 150.

FIG. 4 shows a state where a carrier injection amount control voltage VLof a first level is applied to the first electrode 124 from the carrierinjection amount control signal line Gn while a data signal Vdata isapplied to the gate of the driving transistor 106 and the EL element 102is biased forward. It is assumed that the carrier injection amountcontrol voltage VL has a potential equal to the ground potential. Inthis state, no electron is present in the first electron transfer layer130 a, and the first electron transfer layer 130 a changes to aninsulating state. As a result, the forward current does not flow in theEL element 102, and the EL element 102 is in a light non-emission state.

FIG. 5 shows a state where a carrier injection amount control voltage VHof a second level, which is higher than the first level, is applied tothe first electrode 124 while the EL element 102 is biased forward. Thecarrier injection amount control voltage VH of the second level is apositive voltage, and the electric field formed by the first electrode124 acts to cause the electrons to drift from the first electrontransfer layer 130 a toward the light emitting layer 134.

Since the electric field generated by the first electrode 124 acts onthe first electron transfer layer 130 a, the electrons injected from thesecond electrode 128 into the first electron transfer layer 130 a aredrifted from the peripheral region of the first electron transfer layer130 a toward the central region of the light emitting region 148.

Since the EL element 102 is biased forward, the carriers (electrons)transferred toward the central region of the light emitting region 148are transferred from the first electron transfer layer 130 a toward thelight emitting layer 134. The holes injected from the third electrode140 and the electrons injected from the second electrode 128 arerecombined in the light emitting layer 134 to generate excitons. In thelight emitting layer 134, photons are released when the excitons in anexcited state are transited into a ground state, and as a result, thelight is emitted (light emission state).

In the state where the EL element 102 is biased forward and the carrierinjection amount control voltage VH of the second level is applied tothe first electrode 124, the amount of the electrons injected into thefirst electron transfer layer 130 a is controlled by the level of thevoltage of the second electrode 128. The amount of the electronsinjected into the first electron transfer layer 130 a may be increasedby increasing the voltage of the second electrode 128. The amount of theelectrons injected from the first electron transfer layer 130 a into thelight emitting layer 134 may be controlled by the level of the voltageof the first electrode 124. The voltage of the first electrode 124 maybe increased, so that the electrons injected from the second electrode128 are drawn to the light emitting region 148 in an increased amountand thus the amount of the carriers to be injected into the lightmitting region 148 is increased.

As can be seen, the EL element 102 includes the second electrode 128 asthe negative electrode and the third electrode 140 as the positiveelectrode, and also includes the first electrode 128 applying a voltageindependently from the second electrode 128 and the third electrode 140,so as to control the concentration of the carriers to be injected intothe light emitting layer 134.

In order to allow the light emitting region 148 to emit light at auniform intensity, it is preferred that the electrons flowing in thesecond electron transfer layer 130 b form a space charge limitedcurrent. In order to allow the space charge limited current to flow inthe second electron transfer layer 130 b, it is preferred that the oxidesemiconductor layer forming the second electron transfer layer 130 b isin an amorphous state, a nano-sized crystalline state, or a mixed statethereof. It is preferred that the first electron transfer layer 130 a isa film containing nano-sized crystals and having a high density.

1-2-2. Band Diagram of the EL Element

FIG. 6 shows an example of energy band structure of the EL element 102.The energy band diagram shown in FIG. 6 is of a structure in which thefirst electrode 124, the first insulating layer 126, the first electrontransfer layer 130 a, the second electron transfer layer 130 b, theelectron injection layer 132, a hole blocking layer 133, the lightemitting layer 134, an electron blocking layer 135, the hole transferlayer 136, the hole injection layer 138 and the third electrode 140 arestacked from the left. In the energy band diagram shown in FIG. 6, thesecond electrode 128 is omitted.

The energy band diagram shown in FIG. 6 is of a case where the firstelectrode 124 is formed of a transparent conductive film of indium tinoxide (ITO) or the like, the third electrode 140 is formed of a metalfilm of aluminum or the like, and the first electron transfer layer 130a and the second electron transfer layer 130 b are formed of an oxidesemiconductor. The oxide semiconductor forming the first electrontransfer layer 130 a and the oxide semiconductor forming the secondelectron transfer layer 130 b have different compositions from eachother, and the second electron transfer layer 130 b has a bandgap (Eg2)of a value larger than that of a bandgap (Eg1) of the first electrontransfer layer 130 a. The second electron transfer layer 130 b has athickness greater than that of the first electron transfer layer 130 a.On the basis of the vacuum level, the second electron transfer layer 130b has a lowest unoccupied molecular orbital (LUMO) shallower than thatof the first electron transfer layer 130 a.

While the EL element 102 is biased forward, the electrons injected fromthe second electrode 128 (not shown) into the first electron transferlayer 130 a are drifted toward the light emitting layer 134, whereas theholes injected from the third electrode 140 into the hole injectionlayer 138 are drifted toward the light emitting layer 134. The energylevel of the lowest unoccupied molecular orbital (LUMO) of the electronblocking layer 135 provided adjacent to the light emitting layer 134 isshallower than the energy level of the lowest unoccupied molecularorbital (LUMO) of the light emitting layer 134, and thus the electronsinjected into the light emitting layer 134 are prevented from runningthrough the electron blocking layer 135 toward the hole transfer layer136. The energy level of the lowest unoccupied molecular orbital (LUMO)of the hole blocking layer 133 provided adjacent to the light emittinglayer 134 is deeper than the energy level of the lowest unoccupiedmolecular orbital (LUMO) of the light emitting layer 134, and thus theholes injected into the light emitting layer 134 are prevented fromrunning through the hole blocking layer 133 toward the electroninjection layer 132.

1-2-3. Light Emitting Region of the EL Element

FIG. 7 shows an example of energy band structure of a light emittinglayer 902, a hole blocking layer 904 provided on the negative electrodeside of the light emitting layer 902 and an electron blocking layer 906provided on the positive electrode side of the light emitting layer 902.The EL element emits light by the electrons and the holes beingrecombined in the light emitting layer 902. Specifically, the electronsand the holes are recombined in the light emitting layer 902 to generateexcitons, and the generated excitons are subjected to radiativedeactivation to emit light. In order to increase the current efficiency(light emission efficiency), it is ideal that the electrons and theholes are distributed uniformly to emit light in the entirety of thelight emitting layer 902. A reason for this is that light emissionoccurring in the entirety of the light emitting layer 902 is expected toincrease the current efficiency (light emission efficiency). Inactuality, it is analyzed that a region that actually emits light in thelight emitting layer 902 has a thickness of about 10 nm. However, if thelight emitting layer 902 is formed to have a thickness of 10 nm, thecurrent efficiency (light emission efficiency) is decreased. Therefore,the light emitting layer 902 is practically formed to have a thicknessof about 30 nm to about 50 nm. As shown in FIG. 7, it is expected toincrease the probability of recombination by holding the light emittinglayer 902 between the electron blocking layer 906 and the hole blockinglayer 904 and confining the electrons and the holes in the lightemitting layer 902. However, the mobility of the electrons and the holesin the light emitting layer 902 is as small as about 10⁻⁵cm²/V·sec toabout 10⁻²cm²/V·sec. In addition, the mobility of the holes is severalto several ten times larger than the mobility of the electrons, andtherefore, the distribution of the electrons and the holes isnon-uniform in the light emitting layer 902. For this reason, thecarrier balance (balance between the electrons and the holes) isdestroyed in the light emitting layer 902, and as a result, the currentefficiency (light emission efficiency) is decreased. When the carrierbalance is destroyed and the number of the holes becomes excessive inthe light emitting layer 902, the holes are accumulated on the negativeelectrode side of the light emitting layer 902, and as a result, thecurrent efficiency (light emission efficiency) is decreased. It isconceivable to inject an excessive number of electrons into the lightemitting layer 902 so as to prevent the accumulation of an excessivenumber of holes. However, if the light emission is stopped in the statewhere the electrons remain in the light emitting layer 902, the lightemitting layer 902 is oxidized and progressively deteriorated.

In order to prevent the deterioration of the EL element, the balance ofthe electrons and the holes injected into the light emitting layer needsto be controlled. Namely, if the balance between the amounts of thecarriers to be injected is controlled, the decrease in the currentefficiency (light emission efficiency) is prevented. Although theelement structure of the EL element may be adjusted to temporarilycontrol the carrier balance, there is a problem that the elementstructure, once determined, cannot be changed in accordance with thedeterioration of the EL element or the temperature change. Therefore,regarding the conventional EL element, it is theoretically considered tobe difficult to use the entirety of the light emitting layer to emitlight even though the element structure is optimized.

For an EL element that emits light of a blue wavelength region, theelement structure is designed such that the amount of the electronsinjected into the light emitting layer is larger than the amount of theholes injected into the light emitting layer, with a material having anelectron mobility higher than a hole mobility being selected as a hostmaterial of the light emitting layer. Therefore, as shown in FIG. 7, thelight emitting layer 902 has light emission positions (positions in athickness direction) that are concentrated in the vicinity of the borderwith the light blocking layer 906. In this case, even if the value ofthe current flowing in the EL element is small, the exciton formationconcentration may be increased in a region of the light emitting layer902 that is in the vicinity of the border with the light blocking layer906. As a result, the TTF (Triplet-Triplet Fusion) phenomenon that asinglet exciton is generated by fusion of two triplet excitons may beexpressed to increase the light emission efficiency. As shown in FIG. 7,in the structure of the conventional two-terminal diode-type EL element,the light emission positions in the light emitting layer 902 (positionin the thickness direction) are concentrated in the vicinity of theborder with the light blocking layer 906 in any of a low current region,a medium current region and a high current region. In this case, theexcitons generated in the light emitting layer 902 are diffused into thelight blocking layer 906, and the deterioration of the light blockinglayer 906 is promoted. Namely, the structure of the conventional ELelement has a problem that the light emission positions in the lightemitting layer 902 (position in the thickness direction) are fixed andthus cannot be controlled.

1-2-4. Control on the Carrier Balance and Control on the Light EmissionPositions

In order to increase the current efficiency (light emission efficiency)of the EL element and to suppress the deterioration in the luminance ofthe EL element, it is preferred to control the amounts of the electronsand the holes to be injected into the light emitting layer to realize agood balance therebetween.

The EL element 102 according to this embodiment may directly control, bythe first electrode 124, the amount of the electrons to be injected intothe light emitting layer 134. The EL element 102 may control, by thefirst electrode 124, the amount of the electrodes to be transferred,such that the amount of the electrons to be transferred from the secondelectrode 128 to the light emitting layer 134 is not insufficient ascompared with the amount of the holes to be transferred from the thirdelectrode 140 to the light emitting layer 134. In this manner, the ELelement 102 may control the such that the amount of the holes is notexcessive in the light emitting layer 134 or such that the amount of theelectrons is not insufficient in the light emitting layer 134. In otherwords, the EL element 102 controls, by the first electrode 124, theelectron current such that the magnitude of the electron current flowingfrom the second electrode 128 to the light emitting layer 134 is equalto the magnitude of the hole current flowing from the third electrode140 to the light emitting layer 134. In this manner, the EL element 102controls the carrier balance in the light emitting layer 134.

FIG. 8 is a graph, regarding the EL element 102, schematically showingthe relationship between the voltage (Vg) applied to the first electrode124 and the current (Ie) flowing between the third electrode 140 and thesecond electrode 128, with the voltage (Vac) applied between the thirdelectrode 140 and the second electrode 128 being constant. As shown inFIG. 8, when the voltage (Vg) applied to the first electrode 124 is 0 V,the magnitude of the current (Ie) is small and the light emission of theEL element 102 is not observed. When the voltage applied to the firstelectrode 124 is increased from this state in the positive direction,the carriers (electrons) injected from the second electrode 128 into theelectron transfer layer 130 form a current flowing toward the lightemitting layer 134. At this point, the current (Ie) is exponentiallyincreased like the forward current of a diode (“I region” shown in FIG.8).

When the voltage (Vg) applied to the first electrode 124 is furtherincreased, the amount of increase in the current (Ie) as compared withthe amount of change in the voltage (Vg) tends to be saturated, and thegradient of the Ie vs. Vg characteristic curve becomes mild (“II region”shown in FIG. 8). In the II region, when the level of the voltage (Vg)applied to the first electrode 124 is changed between a first voltage(Vg1) and a second voltage (Vg2), the magnitude of the current (Ie) ischanged between a first current (Ie1) and a second current (Ie2). Aregion where the voltage (Vg) of the first electrode 124 is changedbetween the first voltage (Vg1) and the second voltage (Vg2) is a regionwhere the magnitude of the current (Ie) is not rapidly changed, and is aregion where the light emission intensity of the EL element 102 isbecoming saturated.

The change in the magnitude of the current (Ie) flowing in the ELelement 102 indicates an increase or a decrease in the amount of theelectrons injected into the light emitting layer 134. When the voltage(Vg) of the first electrode 124 is changed between the first voltage(Vg1) and the second voltage (Vg2), the amount of the electrons injectedinto the light emitting layer 134 is changed. Namely, the carrierbalance between the electrons and the holes in the light emitting layer134 may be controlled by changing the voltage (Vg) of the firstelectrode 124. When the amount of the electrons to be injected into thelight emitting layer 134 is changed, the central position of the regionwhere the electrons and the holes are recombined (position in the lightemitting region in the thickness direction of the light emitting layer134) may be shifted. For example, when the first electrode 124 has thefirst voltage (Vg1), the magnitude of the electron current is relativelysmall as compared with the magnitude of the hole current, and the lightemission positions in the light emitting layer 134 are on the negativeelectrode side (“A” side in FIG. 8). By contrast, when the firstelectrode 124 has the second voltage (Vg2), the magnitude of theelectron current is relatively large as compared with the magnitude ofthe hole current, and the light emission positions in the light emittinglayer 134 are shifted to the positive electrode side (“B” side in FIG.8).

As described above, the EL element 102 may control, by the voltage ofthe first electrode 124, the light emission positions, in the thicknessdirection, in the light emitting layer 134. For example, when thevoltage of the first electrode 124 is changed between the first voltage(Vg1) and the second voltage (Vg2), the light emission positions in thelight emitting layer 134 may be moved between the negative electrodeside A and the positive electrode side B. The entirety of the lightemitting layer 134 may be used as the light emitting region bycontrolling the voltage of the first electrode 124. Since the entiretyof the light emitting layer 134 is used as the light emitting region,the life until the luminance is deteriorated (e.g., time until theluminance is decreased to 70% of the initial luminance) may be extended.The voltage of the first electrode 124 may be changed between Vg1 andVg2 shown in FIG. 8, and the level of the luminance may be controlled bythe potential difference (voltage) between the second electrode 128 andthe third electrode 140.

As described above, in the EL element 102 according to this embodiment,the first electrode 124 controlling the amount of the carriers to beinjected is located to face the electron transfer layer 130 with thefirst insulating layer 126 being located between the first electrode 124and the electron transfer layer 130, and the first electrode 124 islocated also to face the third electrode 140, which is the positiveelectrode. With such a structure, the EL element 102 may control theamount of the electrons to be injected into the light emitting layer134. The EL element 102 according to this embodiment may control thecarrier balance between the electrons and the holes in the lightemitting layer 134 by the action of the first electrode 124 controllingthe amount of the carriers to be injected. In this manner, the ELelement 102 may increase the current efficiency thereof and extend thelife thereof.

By contrast, in the EL element disclosed in Japanese Laid-Open PatentPublication No. 2002-343578, the light emitting material layer has a lowelectron mobility. Therefore, the amount of the electrons to be injectedfrom the negative electrode is substantially determined by the potentialdifference between the positive electrode and the negative electrode,and the bias voltage applied by the auxiliary electrode does notinfluence the carrier injection almost at all. For this reason, the ELelement disclosed in Japanese Laid-Open Patent Publication No.2002-343578, even if used in a pixel in a display device, does not emitlight uniformly in the pixel plane. In the light emitting transistordisclosed in WO2007/043697, the auxiliary electrode controls the lightemission/light non-emission state. Therefore, the amounts of theelectrons and the holes to be injected into the light emitting layercannot be controlled independently. In addition, the light emittingtransistor disclosed in WO2007/043697 cannot control the position of theregion where the electrons and the holes are recombined in the lightemitting layer, namely, the position of the light emitting region. Inthe organic light emitting transistor disclosed in each of JapaneseLaid-Open Patent Publication No. 2007-149922 and Japanese

Laid-Open Patent Publication No. 2007-157871, the electron transferlayer formed of an organic material has a low carrier (electron)mobility. Therefore, the display screen cannot be large or of highdefinition. The electron transfer layer formed of an organic material asdescribed in each of Japanese Laid-Open Patent Publication No.2007-149922 and Japanese Laid-Open Patent Publication No. 2007-157871has a carrier (electron) mobility of 2.5 cm²/V·sec or lower. For thisreason, it is considered to be difficult to make the display panel largeor of high definition.

In addition, in the structure of the conventional EL element, the lightemitting layer is not entirely deteriorated uniformly in the thicknessdirection, but is deteriorated non-uniformly. Therefore, it is difficultto suppress the deterioration in the luminance, and thus the life of theEL element cannot be extended. By contrast, the EL element 102 accordingto this embodiment allows the entirety of the light emitting layer 134to act as the light emitting region by controlling the voltage of thefirst electrode 124, and therefore, may extend the life until theluminance is deteriorated as compared with the conventional EL element.Even if the thickness of the light emitting layer 134 is increased to 45nm to 90 nm, which is 1.5 times to 3 times the thickness of theconventional EL element (e.g., 30 nm), the entirety of the lightemitting layer 134 in the thickness direction may emit light, and thelife of the EL element 102 may be extended.

2. Display Device Including the EL Element

A display device including the EL element has a use such as a TV, amobile information terminal (e.g., a mobile phone including aninternet-connectable OS (operating system) called a “smartphone”) or thelike.

Hereinafter, an example of display device including the EL element 102 aof the bottom-emission type shown in FIG. 1 or the EL element 102 b ofthe top-emission type shown in FIG. 2 will be described.

2-1. Circuit Configuration of the Display Device Including the ELElement (EL Display Device)

FIG. 9 shows a structure of an EL display device 100 according to anembodiment of the present invention. The EL display device 100 includesthe substrate 110, and also includes a display 112 including an array ofa plurality of pixels 114, a scanning signal line driving circuit block116 outputting a scanning signal to scanning signal lines Sn, a datadriver IC 118 outputting a data signal to data signal lines Dm, and acarrier injection amount control signal line driving circuit block 120outputting a control signal to the carrier injection amount controlsignal lines Gn, which are provided on the substrate 110.

The display 112 includes the scanning signal lines Sn, the data signallines Dm, the carrier injection amount control signal lines Gn andcommon lines Cn provided therein. The plurality of pixels 114 is eachelectrically connected with one of the scanning signal lines Sn, one ofthe data signal lines Dm, one of the carrier injection amount controlsignal lines Gn and one of the common lines Cn. Although not shown indetail in FIG. 9, the plurality of pixels 114 each include an EL elementand a transistor controlling the light emission of the EL element. Asshown in FIG. 1 and FIG. 2, the EL element is a three-terminal elementincluding the first electrode (carrier injection amount controlelectrode) 124, the second electrode (negative electrode) 128 and thethird electrode (positive electrode) 140.

2-1-1. Equivalent Circuit of the Pixel

FIG. 10 shows an example of equivalent circuit of the pixel 114. Thepixel 114 includes the EL element 102, a selection transistor 104, thedriving transistor 106, and the capacitive element 108. FIG. 10schematically shows the positional arrangement of the electrodes insteadof using circuit symbols, in order to show the connection structure ofthe EL element 102. FIG. 10 schematically shows the structure of the ELelement 102 as a stack of the first electrode 124, the second electrode128, the third electrode 140, and also the first insulating layer 126,the electron transfer layer 130 (the first electron transfer layer 130 aand the second first electron transfer layer 130 b) and the lightemitting layer 134 between the first electrode 124 and the thirdelectrode 140.

The selection transistor 104 includes a gate electrically connected withthe scanning signal line Sn, a source electrically connected with thedata signal line Dm and a drain electrically connected with a gate ofthe driving transistor 106. The driving transistor 106 includes a sourceelectrically connected with the common line Cn and a drain electricallyconnected with the second electrode 128 of the EL element 102. Thecapacitive element 108 is electrically connected between the gate of thedriving transistor 106 and the common line Cn. Regarding the EL element102, the first electrode 124 is connected with the carrier injectionamount control signal line Gn, the second electrode 128 is connectedwith the drain of the driving transistor 106, and the third electrode140 is connected with the power supply line PL. As represented by thedashed line in FIG. 10, the selection transistor 104 and the drivingtransistor 106 may each have a dual-gate structure. The common line Cnprovided parallel to the carrier injection amount control signal line Gnand the scanning signal line Sn may be electrically connected with acommon line Cm in a region where the common lines Cn and Cm cross eachother. The common line Cm is provided parallel with the data signal lineDm.

The scanning signal line Sn is supplied with a scanning signalcontrolling the selection transistor 104 to be on or off. The datasignal line Dm is supplied with a data signal (video signal). The commonlines Cm and Cn are kept at a constant potential (e.g., groundpotential). The power supply line PL is supplied with a supply voltage(VDD). The carrier injection amount control signal line Gn is suppliedwith a carrier injection amount control signal controlling the amount ofthe carriers to be injected. As described below, the carrier injectionamount control signal is not of a constant voltage, but is of a voltagehaving a level changing along with the time.

FIG. 11 shows an equivalent circuit of the EL element 102 representedwith circuit symbols, and the circuit configuration shown in FIG. 11 isthe same as that in FIG. 10. The EL element 102 is a three-terminalelement. The second electrode (negative electrode) 128 is connected withthe drain of the driving transistor 106, the third electrode (positiveelectrode) 140 is connected with the power supply line PL, and the firstelectrode (carrier injection amount control signal) 124 is connectedwith the carrier injection amount control signal line Gn. The circuitsymbols shown in FIG. 11 show that the EL element 102 is athree-terminal element including the first electrode 124, the secondelectrode 128 and the third electrode 140.

As shown in FIG. 1 and FIG. 2, the EL element 102 includes the offsetregion 150. The offset region 150 overlaps the first electrode 124 withthe first insulating layer 126 being provided between the offset region150 and the first electrode 124. The amount of the carriers (electrons)flowing in the offset region 150 is controlled by the first electrode124. Because of such a structure, the EL element 102 may be regarded asincluding a parasitic transistor, or may be regarded as a compositeelement of a diode and a transistor. The circuit symbols used for the ELelement 102 in FIG. 11 indicate that the EL element 102 hassubstantially the same structure as that of a transistor.

The equivalent circuit shown in FIG. 10 and FIG. 11 operates as follows.When the gate of the selection transistor 104 is supplied with ascanning signal from the scanning signal line Sn, the selectiontransistor 104 is turned on, and a voltage based on a data signal isapplied to the gate of the driving transistor 106 from the data signalline Dm. The capacitive element 108 is charged with the voltage of thedata signal line Dm, and temporarily holds the data signal. When thedriving transistor 106 becomes conductive, a current flows from thepower supply line PL into the EL element 102. The current to flow intothe EL element 102 is controlled by the gate voltage of the drivingtransistor 106. The current to flow into the EL element 102 is alsocontrolled by the first electrode 124. The current to flow into the ELelement 102 is controlled by the driving transistor 106 and the firstelectrode 124 independently.

2-1-2. Waveform of the Carrier Injection Amount Control Signal

FIG. 12 shows an example of voltage waveforms of signals applied to thescanning signal line Sn, the carrier injection amount control signalline Gn and the data signal line Dm and an example of waveform of avoltage Vst of the capacitive element 108. FIG. 12 indicates that oneframe period F includes a horizontal period 1H in which the scanningsignal line Sn is supplied with a scanning signal and a light emissionperiod T in which the EL element 102 emits light. In the horizontalperiod 1H, when a scanning signal (Vscn) is applied to the scanningsignal line Sn, the selection transistor 104 is turned on, a data signal(data voltage Vdata) is applied from the data signal line Dm to the gateof the driving transistor 106, and at the same time, the capacitiveelement 108 is charged with the data voltage Vdata. The voltage Vstcharging the capacitive element 108 is changed in accordance with thedata voltage Vdata. Even after the selection transistor 104 is turnedoff, the capacitive element 108 is charged with the data voltage Vdata.Therefore, the gate voltage of the driving transistor 106 is maintained.

In the horizontal period 1H, the carrier injection amount control signalline Gn is supplied with a carrier injection amount control signal Vg(Vg=VL=0 V) of a first level. Therefore, even if the gate of the drivingtransistor 106 is supplied with a data signal (signal of a voltage of alevel higher than the level of a threshold voltage of the drivingtransistor 106) and a forward voltage is applied between the secondelectrode (negative electrode) 128 and the third electrode (positiveelectrode) 140 of the EL element 102, the light non-emission state ismaintained.

When the horizontal period 1H is finished, the EL element 102 istransited to the light emission period T. During the light emissionperiod T, the carrier injection amount control signal line Gn issupplied with a carrier injection amount control signal Vg having avoltage of a level higher than the first level. As a result of the firstelectrode (carrier injection amount control electrode) 124 beingsupplied with the carrier injection amount control signal Vg, the ELelement 102 emits light. The voltage of the carrier injection amountcontrol signal Vg may be maintained at a constant level, or may be amodulation voltage, the level of which is changed along with the time asshown in FIG. 12.

The carrier injection amount control signal Vg shown in FIG. 12 has avoltage waveform that is changed from V1 to V2 to V3 as the time elapsesfrom t1 to t2 to t3. The voltages of the carrier injection amountcontrol signal Vg have the relationship of V1>V2>V3, and therelationship of (V1, V2, V3)>VL. Namely, the carrier injection amountcontrol signal Vg has a voltage waveform that decreases in a step-likemanner from the first voltage V1 to the second voltage V2 to the thirdvoltage V3 in the light emission period T. The voltage of the carrierinjection amount control signal Vg may be changed in this manner, sothat the amount of the carriers to be injected into the light emittinglayer 134 is controlled and the light emission positions in the lightemitting layer 134 are controlled. When the light emission period T isfinished, the voltage of the carrier injection amount control signal Vgis changed to the voltage VL of the first level. As a result, the lightemission of the EL element 102 is stopped. In this manner, the level ofthe voltage of the carrier injection amount control signal Vg to beapplied to the carrier injection amount control signal line Gn may becontrolled, so that the EL element 102 is controlled to be in the lightemission state or in the light non-emission state, and the lightemission positions in the light emitting layer 134 are controlled.

It is preferred to set the voltage of the carrier injection amountcontrol signal Vg as follows. The voltage V1 is set such that the lightemission positions (position in the thickness direction of the lightemitting layer 134) of the EL element 102 are in a positiveelectrode-side light emitting region EL(t), the voltage V2 is set suchthat the light emission positions are in a central light emitting regionEL(m), and the voltage V3 is set such that the light emission positionsare in a negative electrode-side light emitting region EL(b). Thesevalues of the voltages are examples, and the values of the voltages ofthe carrier injection amount control signal Vg may be appropriately setin accordance with the structure of the EL element 102.

FIG. 13A, FIG. 13B, FIG. 14A and FIG. 14B show other examples of thevoltage waveform of the carrier injection amount control signal Vg. Asshown in FIG. 13A, the carrier injection amount control signal Vg mayhave a voltage waveform that increases in a step-like manner from V3 toV2 to V1 as the time elapses from t1 to t2 to t3 in the light emissionperiod T. As shown in FIG. 13B, the carrier injection amount controlsignal Vg may have a voltage waveform that continuously changes from V1to V2 in time period t1 in the light emission period T, is kept at V2 intime period T2, and continuously changes from V2 to V3 in time period T3(V1>V2>V3). Although not shown, in a modification of FIG. 13, thecarrier injection amount control signal Vg may have a voltage waveformthat increases from V3 to V1 as the time elapses in the light emissionperiod T. FIG. 14A and FIG. 14B show examples in which the voltagewaveform of the carrier injection amount control signal Vg changes in asine wave manner. FIG. 14A shows a waveform of the voltage Vg of thecarrier injection amount control signal that continuously increases fromV3 to V2 to V1 and then continuously decreases from V1 to V2 to V3 inthe light emission period T. FIG. 14B shows a waveform of the voltage Vgof the carrier injection amount control signal that continuouslydecreases from V1 to V2 to V3 and then continuously increases from V3 toV2 to V1 in the light emission period T. As can be seen, the voltagewaveform of the carrier injection amount control signal Vg may bechanged, so that the light emission positions in the light emittinglayer 134 are controlled and the state of deterioration is made uniformin the light emitting layer 134.

The EL element, in the case of being formed of a phosphorescent materialor a thermally activated delayed fluorescence (TADF) material, emitslight in a central region of the light emitting layer 134, instead of inthe vicinity of the interface between the light emitting layer 134 and alayer adjacent thereto. In this manner, the light emission efficiencymay be increased, and the life until the luminance is deteriorated maybe extended. Therefore, the period in which the carrier injection amountcontrol signal Vg of an intermediate voltage level is applied may beextended. For example, in the case of the example of FIG. 12, the timeperiod t2 may be extended (t2≥t1, t3).

For a light emitting layer that emits blue light, a fluorescent materialis used as a light emitting material. Therefore, in order to increasethe light emission efficiency of the EL element and extend the lifethereof, the light emitting region needs to be formed in a concentratedmanner in the vicinity of the positive electrode-side interface of thelight emitting layer 134 or in the vicinity of the negativeelectrode-side interface of the light emitting layer 134 to express theTTF phenomenon. Therefore, for a single light emitting layer 134emitting blue light, it is preferred to extend the time period t1 or thetime period t3. In general, the light emission efficiency and the lengthof the life are superior in the case where the light emission positions(positions in the thickness direction) are concentrated in the vicinityof the positive electrode-side interface of the light emitting layer134. For this reason, it is preferred to extend the time period t1, inwhich the voltage V1, at which the amount of the electrons to beinjected may be maximized, is applied.

However, if the light emission positions are kept on concentrated onlyin the vicinity of the positive electrode-side interface of the lightemitting layer 134, the deterioration of the electron blocking layer (orthe hole transfer layer) adjacent to the light emitting layer 134 ispromoted. In order to prevent such deterioration, the EL element 102according to an embodiment of the present invention may change thevoltage of the carrier injection amount control signal Vg to be appliedto the first electrode (carrier injection amount control electrode) 124to move the light emission positions in the light emitting layer 134.Alternatively, the hole blocking layer may be provided at the negativeelectrode-side interface of the light emitting layer 134, so that thevoltage of the carrier injection amount control signal Vg is decreasedto move the light emission positions in the light emitting layer 134 tothe vicinity of the negative electrode-side (electron transferlayer-side) interface. As a result, the TTF phenomenon may be expressedalso in the vicinity of the negative electrode-side (electron transferlayer-side) interface of the light emitting layer 134. Thus, the lightemission efficiency of the EL element emitting blue light may beincreased in all the current regions, more specifically, the smallcurrent region, the medium current region and the large current region.FIG. 12, FIG. 13A and FIG. 13B show an embodiment in which a set ofcontrol voltages V1, V2 and V3 are applied in one frame period F. Theembodiment of the present invention is not limited to this. As shown inFIG. 15A and FIG. 15B, an application of a set of control voltages V1,V2 and V3 in a step-like manner may be repeated a plurality of times inone frame period F. Alternatively, as shown in FIG. 16A and FIG. 16B, anapplication of a set of control voltages in a sine wave manner may berepeated a plurality of times in one frame period F. In the case where,for example, the frame frequency is 60 Hz, the maximum possible numberof times of repetition may be about 1000 times.

The ratio at which the voltage value of the carrier injection amountcontrol signal Vg is changed may be arbitrarily set. For example, FIG.12 indicates that the voltage value is changed in three steps.Alternatively, the voltage value may be changed in a larger number ofsteps. The voltage value of the carrier injection amount control signalVg may be changed in a large number of steps, so that the light emissionpositions in the light emitting layer 134 are controlled more precisely.Alternatively, the voltage value of the carrier injection amount controlsignal Vg may be changed in two steps. In this case, the light emissionpositions in the light emitting layer 134 may be changed, and also thepower consumption of a carrier injection amount control signal linedriving circuit may be decreased.

2-1-3. Circuit Configuration of the Display

A circuit configuration of the display 112 of the EL element 102 will bedescribed. The circuit configuration of each of the pixels 114 in thedisplay 112 is equivalent to the equivalent circuit shown in FIG. 11.

2-1-3-1. Circuit Configuration 1 of the Display

FIG. 17 shows a circuit configuration of a display 112 a of the ELdisplay device 100. The display 112 a includes the pixels 114 arrayed ina matrix. FIG. 17 shows a first pixel 114(n) located at the n'th row anda certain column and a second pixel 114(n+1) located at the (n+1)'th rowand the certain column. The first pixel 114(n) and the second pixel114(n+1) each have a circuit configuration substantially the same asthat of the pixel 114 described above with respect to FIG. 11.

FIG. 17 also shows an embodiment in which scanning signal lines Sn−1, Snand Sn+1, data signal lines D1 m, D1 m+1, D2 m and D2 m+1, common linesCm, Cm+1, Cn and Cn+1, and carrier injection amount control signal linesGn and Gn+1 located in the display 112 a. The common lines Cm and Cm+1and the common lines Cn and Cn+1 are located in directions crossing eachother, and are electrically connected with each other in an appropriatemanner at positions where the common lines cross each other. The pixels114 are assumed to be arrayed in k rows and j columns. It is alsoassumed that m=1 to j (m1=1 to 0.5×j, m2=1 to 0.5×j), and that n=1 to k.

The first pixel 114(n) includes a selection transistor 104(n), a drivingtransistor 106(n), a capacitive element 108(n), and an EL element102(n). Similarly, the second pixel 114(n+1) includes a selectiontransistor 104(n+1), a driving transistor 106(n+1), a capacitive element108(n+1), and an EL element 102(n+1).

The first pixel 114(n) has the following connection structure. Theselection transistor 104(n) includes a gate electrically connected withthe scanning signal line Sn, a source electrically connected with thedata signal line D1 m, and a drain electrically connected with a gate ofthe driving transistor 106(n). The driving transistor 106(n) includes asource electrically connected with the common line Cn and a drainelectrically connected with the second electrode (negative electrode)128 of the EL element 102(n). The capacitive element 108(n) is connectedbetween the gate of the driving transistor 106(n) and the common lineCn. The EL element 102(n) includes the first electrode (carrierinjection amount control electrode) 124 electrically connected with thecarrier injection amount control signal line Gn and the third electrode(positive electrode) 140 electrically connected with the power supplyline PL.

The second pixel 114(n+1) has the following connection structure. Theselection transistor 104(n+1) include a gate electrically connected withthe scanning signal line Sn+1 and a source electrically connected withthe data signal line D2 m. The driving transistor 106(n+1) includes asource electrically connected with the common line Cn+1. The capacitiveelement 108(n+1) is connected between a gate of the driving transistor106(n+1) and the common line Cn+1. The EL element 102(n+1) includes thefirst electrode (carrier injection amount control electrode) 124electrically connected with the carrier injection amount control signalline Gn+1 and the third electrode (positive electrode) 140 electricallyconnected with the power supply line PL.

In the display 112 a, two data signal lines D1 m and D2 m are arrayed onone column. The selection transistor 104(n) on the n'th row is connectedwith the data signal line D1 m, and the selection transistor 104(n+1) onthe (n+1)'th row is connected with the data signal line D2 m. Such acircuit configuration allows two pixels to be selected at the same timeso that a data signal is written to each of the two pixels.

2-1-3-2. Driving System 1 of the Display

FIG. 18 shows an operation of the first pixel 114(n) and the secondpixel 114(n+1) shown in FIG. 17. In a certain frame period F1, a datasignal is written to the first pixel 114(n) from the data signal line D1m in a horizontal period H11, in which the scanning signal Vscn isapplied to the scanning signal line Sn. After the horizontal period H11is finished, the first pixel 114(n) is transited to a light emissionperiod T11. In the light emission period T11, the first pixel 114(n) issupplied with the carrier injection amount control signal Vg from thecarrier injection amount control signal line Gn, and as a result, emitslight.

The second pixel 114(n+1) is selected at the same time as the firstpixel 114(n). A data signal is written to the second pixel 114(n+1) fromthe data signal line D2 m, and a light emission period T21 is started atthe same time as the light emission period T11. Namely, the data signalis written to the second pixel 114(n+1) from the data signal line D2 min a selection period H21, in which the scanning signal Vscn is appliedto the scanning signal line Sn+1. After the selection period H21 isfinished, the second pixel 114(n+1) is transited to the light emissionperiod T21. In the light emission period T21, the second pixel 114(n+1)is supplied with the carrier injection amount control signal Vg from thecarrier injection amount control signal line Gn+1, and as a result,emits light.

The first pixel 114(n) and the second pixel 114(n+1) are generallyoperated as described above in one frame period F1. Substantially thesame operation is performed in the next frame period F2. Namely, theoperation in which the first pixel 114(n) is transited to a lightemission period T12 after one horizontal period H12 and the second pixel114(n+1) is transited to a light emission period T22 after a onehorizontal period H22 is repeated. Such an operation is performed at thesame time in the pixels arrayed in the row direction along the scanningsignal lines Sn and Sn+1. In FIG. 18, the carrier injection amountcontrol signals Vg applied to the carrier injection amount controlsignal lines Gn and Gn+1 have the waveform substantially the same asthat shown in FIG. 12. The waveform of the carrier injection amountcontrol signals Vg may be replaced with the waveform shown in FIG. 13Aor FIG. 13B.

As shown in FIG. 17, the data signal line D1 m and the data signal lineD2 m may be arrayed on one column, so that two pixels are selected atthe same time and the data signal is written to each of the two pixels.With such a structure, even in the case where the number of thehorizontal lines (number of the scanning signal lines) is increased,there is a sufficient duration of time in which the data signal iswritten to each pixel. The carrier injection amount control signal linesare provided respectively for each row as shown as carrier injectionamount control signal lines Gn and Gn+1, and therefore, light emissionstate of the EL elements may be controlled in all the pixels in thedisplay.

FIG. 18 shows an embodiment in which the data signal Vdata is written totwo scanning signal lines Sn and Sn+1 at the same time in one horizontalperiod 1 H. Since two scanning signal lines are allowed to be selectedat the same time, the period in which the data signal Vdata is writtenmay be extended to two horizontal periods 2H. In this case, there is asufficient duration of time in which the data signal is written to eachpixel.

2-1-3-3. Circuit Configuration 2 of the Display

FIG. 19 shows a circuit configuration of a display 112 b of the ELdisplay device 100. In the display 112 b shown in FIG. 19, the firstpixel 114(n) and the second pixel 114(n+1) each have a structuresubstantially the same as that shown in FIG. 17. The display 112 b shownin FIG. 19 is different from the display 112 a shown in FIG. 17 in thestructures of the scanning signal line Sn, the common line Cn and thecarrier injection amount control signal line Gn. Hereinafter, thedifferences will be described.

The scanning signal line Sn is common to the first pixel 114(n) and thesecond pixel 114(n+1). Namely, the gate of the selection transistor104(n) of the first pixel 114(n) and the gate of the selectiontransistor 104(n+1) of the second pixel 114(n+1) are both connected withthe scanning signal line Sn. The carrier injection amount control signalline Gn is common to the first pixel 114(n) and the second pixel114(n+1). Namely, the first electrode 124 of the EL element 102(n) ofthe first pixel 114(n) is connected with the carrier injection amountcontrol signal line Gn, and the first electrode 124 of the EL element102(n+1) of the second pixel 114(n+1) is also connected with the carrierinjection amount control signal line Gn.

The common line Cn is common to the pixels belonging to the n'th row andthe pixels belonging to the (n−1)'th row. The driving transistor 106(n)and the capacitive element 108(n) of the first pixel 114(n) areconnected with the common line Cn, and the driving transistor 106(n−1)and the capacitive element 108(n−1) of the third pixel 114(n−1) are alsoconnected with the common line Cn. On the other hand, the drivingtransistor 106(n+1) and the capacitive element 108(n+1) of the secondpixel 114(n+1) are connected with the common line C(n+1).

In this manner, the scanning signal line Sn, the carrier injectionamount control signal line Gn and the common line Cn may be provided tobe common to the pixels belonging to two rows adjacent to each other, sothat the number of the lines provided in the display 112 b is decreased.Such a structure simplifies the circuit configurations of the scanningsignal line driving circuit block 116 and the carrier injection amountcontrol signal line driving circuit block 120, in addition to decreasingthe number of the lines provided in the display 112 b. In the case wherethe EL element 102 a of the bottom-emission type is provided in thepixel 114, the numerical aperture of the pixel may be increased.

2-1-3-4. Driving System 2 of the Display

FIG. 20 shows an operation of the first pixel 114(n) and the secondpixel 114(n+1) shown in FIG. 19. In a certain frame period F1, a datasignal is written to each of the first pixel 114(n) and the second pixel114(n+1) respectively from the data signal lines D1 m and D2 m in twohorizontal periods H11, in which the scanning signal Vscn is applied tothe scanning signal line Sn. After the two horizontal periods H11 arefinished, the first pixel 114(n) and the second pixel 114(n+1) aretransited to a light emission period T11. In the light emission periodT11, the first pixel 114(n) and the second pixel 114(n+1) are eachsupplied with the carrier injection amount control signal Vg from thecarrier injection amount control signal line Gn, and as a result, emitlight.

As described above, the first pixel 114(n) and the second pixel 114(n+1)are selected at the same time by the scanning signal line Sn. A datasignal is written to each of the first pixel 114(n) and the second pixel114(n+1) respectively from the data signal lines D1 m and D2 m, and thelight emission period T11 is started at the same time in the first pixel114(n) and the second pixel 114(n+1). As shown in FIG. 19, the pixels oftwo rows are selected by one scanning line Sn, and the data signal Vdatais written to each of the pixels of the two rows at the same time. Withsuch a structure, even in the case where the number of the horizontallines (number of the scanning signal lines) is increased, there is asufficient duration of time in which the data signal is written to eachpixel. In addition, the period in which the data signal Vdata is writtenmay be set to two horizontal periods. In this case, there is asufficient duration of time in which the data signal is written to eachpixel.

2-1-3-5. Driving System 3 of the Display

According to the video format of the international standard, 4 K refersto a resolution of about 8 million pixels (3,840×2,160 pixels), and 8 Krefers to a resolution of about 3,300 million pixels (7,680×4,320pixels). The density of the pixels in a display panel needs to beincreased in correspondence with such a high definition of the video.However, when the number of the horizontal lines (scanning signal lines)is increased in a display panel, there is a problem that if the framefrequency is of a certain constant level, the time duration in which thesignal is written to one horizontal line is shortened. By contrast, ifthe frame frequency is decreased, there is a problem that the quality ofa moving image is decreased.

In a display device, an image is displayed by rewriting an image onceevery frame. It is considered in principle that a smoother moving imageis displayed when the frame frequency is higher. Specifically, it isconsidered that a frame frequency of 120 Hz or 240 Hz is more suitableto reproduce a moving image than a frame frequency of 60 Hz.

Even in the case where the frame frequency is 60 Hz, a driving systemshown in FIG. 21 provides moving image characteristics equivalent tothose of the case where the frame frequency of 120 Hz. According to thedriving system shown in FIG. 21, a display screen 908 has a length L1 ina vertical direction, and a length L2 of an image display region 910 inthe vertical direction is half of the length L1 (L2=0.5×L1). In oneframe period, the image display region 910 of the display screen 908 isscanned once from the top to the bottom. In the case where the length L2of the image display region 910 in the vertical direction is about ¼ ofthe length L1 of the display screen 908 in the vertical direction(L2=0.25×L1), moving image characteristics equivalent to those in thecase where the frame frequency is 240 Hz are provided. Namely, even inthe case where the frame frequency is 60 Hz, the moving imagecharacteristics that are the same as those of the case where the framefrequency is 120 Hz may be provided by setting one frame period to 16.6msec and setting the light emission period to 8.3 msec. Even in the casewhere the frame frequency is 60 Hz, the moving image characteristicsthat are the same as those of the case where the frame frequency is 240Hz may be provided by setting one frame period to 16.6 msec and settingthe light emission period to 4.16 msec.

FIG. 22 shows the waveform of the carrier injection amount controlsignal Vg in the case where the driving system of scanning the imagedisplay region 910 of the display screen 908 in one frame period, asdescribed above is adopted. In one horizontal period 1H, the scanningsignal Vscn is applied to the scanning signal line Sn, and as a result,the selection transistor 104 is turned on and the data signal Vdata iswritten to the gate of the driving transistor 106. After the onehorizontal period 1H is finished, the pixel 114 is transited to a lightemission period T. The light emission period T is set to be shorter thanone frame period F. In the case where, for example, the frame frequencyis 60 Hz and the one frame period F is 16.6 msec, the light emissionperiod T is set to 8.3 msec. With such settings, moving imagecharacteristics equivalent to those of the case where the framefrequency is 120 Hz may be provided. In the case where the lightemission period T is set to 4.16 msec, moving image characteristicsequivalent to those of the case where the frame frequency is 240 Hz maybe provided.

In this case also, as shown in FIG. 22, the voltage of the carrierinjection amount control signal Vg may be changed in a step-like mannerfrom V1 to V2 to V3 as the time elapses from t1 to t2 to t3 in the lightemission period T, so that the light emission positions in the lightemitting layer are controlled as described above. When the lightemission period T is finished, the voltage of the carrier injectionamount control signal Vg may be changed to the voltage VL of the firstlevel (VL=0 V), so that the light emission of the EL element 102 isstopped before the one frame period F is finished. The waveform of thecarrier injection amount control signal Vg may be substantially the sameas that shown in FIG. 13A or FIG. 13B.

As described above, even in the case where the driving system ofscanning 134 are controlled the image display region 910 of the displayscreen 908 in one frame period is adopted, the waveform of the carrierinjection amount control signal Vg may be controlled, so that the lightemission positions in the light emitting layer and thus the luminance ofthe EL element 102 is suppressed from being deteriorated. In thismanner, the light emission period T in one frame period F may be set tobe short and the waveform of the carrier injection amount control signalmay be controlled, so that even when the frame frequency is 60 Hz, animage having superb moving image characteristics are displayed and ahighly reliable display device is provided.

2-1-3-6. Circuit Configuration 3 of the Display

FIG. 23 shows a circuit configuration of a display 112 c of the ELdisplay device 100. FIG. 23 shows a configuration in which pixelscorresponding to the same color are arrayed in the row direction.Specifically, FIG. 23 shows an embodiment in which a first R pixel 114r(m) and a second R pixel 114 r(m+1) corresponding to red are providedon the n'th row, a first G pixel 114 g(m) and a second G pixel 114g(m+1) corresponding to green are located on the (n+1)'th row, and afirst B pixel 114 b(m) and a second B pixel 114 b(m+1) corresponding toblue are located on the (n+2)'th row.

The structure of each pixel is substantially the same as that in FIG.17. For example, the first R pixel 114 r(m) includes an EL element 102r(m), a selection transistor 104 r(m), a driving transistor 106 r(m),and a capacitive element 108 r(m). The EL element 102 r(m) of the firstR pixel 114 r(m) includes a first electrode (carrier injection amountcontrol electrode) connected with a carrier injection amount controlsignal line Gn_r, and the EL element 102 r(m+1) of the second R pixel114 r(m+1) includes a first electrode (carrier injection amount controlelectrode) also connected with the carrier injection amount controlsignal line Gn_r. An EL element 102 g(m) of the first G pixel 114 g(m)includes a first electrode (carrier injection amount control electrode)connected with a carrier injection amount control signal line Gn+1_g,and an EL element 102 g(m+1) of the second G pixel 114 g(m+1) includes afirst electrode (carrier injection amount control electrode) alsoconnected with the carrier injection amount control signal line Gn+1_g.An EL element 102 b(m) of the first B pixel 114 b(m) includes a firstelectrode (carrier injection amount control electrode) connected with acarrier injection amount control signal line Gn+2_b, and an EL element102 b(m+1) of the second B pixel 114 b(m+1) includes a first electrode(carrier injection amount control electrode) also connected with thecarrier injection amount control signal line Gn+2_b.

The first R pixel 114 r(m), the first G pixel 114 g(m) and the first Bpixel 114 b(m) located on the m'th column respectively include ELelements emitting light of different colors. For example, the EL element102 r(m) includes a light emitting layer formed of a light emittingmaterial that emits red light. The EL element 102 g(m) includes a lightemitting layer formed of a light emitting material that emits greenlight. The EL element 102 b(m) includes a light emitting layer formed ofa light emitting material that emits blue light. The same is applicableto the second R pixel 114 r(m+1), the second G pixel 114 g(m+1) and thesecond B pixel 114 b(m+1) located on the (m+1)'th column.

An EL element including a light emitting layer formed of a differentmaterial exhibits a different timewise change (deterioration) in thecurrent-voltage characteristic and the current efficiency. For thisreason, as shown in FIG. 24, a display 112 d includes the carrierinjection amount control signal lines Gn in accordance with the ELelements emitting light of different colors. With such a structure, adifferent carrier injection amount control voltage may be applied to thelight emitting layer of the EL element that emits light of a differentcolor. FIG. 24 shows an embodiment in which the carrier injection amountcontrol signal line Gn_r is provided for the first R pixel 114 r(n)located on the n'th row, the carrier injection amount control signalline Gn_g is provided for the first G pixel 114 g(n) located on the n'throw, and the carrier injection amount control signal line Gn_b isprovided for the first B pixel 114 b(n) located on the n'th row. In thismanner, a plurality of carrier injection amount control signal lines maybe provided for each row, namely, may be provided in correspondence withthe colors of light, so that the carrier injection amount of the ELelement that emits light of each color is independently controlled. Inthe case where, for example, the deterioration rate of the currentefficiency of the EL element 102 b(n) emitting blue light is higher thanthat of the EL element 102 r(n) emitting red light, the carrierinjection amount control voltage to be applied to the carrier injectionamount control signal line Gn_b connected with the EL element 102 b(n)emitting blue light may be made higher than the carrier injection amountcontrol voltage to be applied to the carrier injection amount controlsignal line Gn_r connected with the EL element 102 r(n) emitting redlight, in consideration of the deterioration in the current efficiency.In this manner, the optimal light emission positions at which the lightemission efficiency may be increased may be controlled, so that thedeterioration in the image quality is prevented and the life of thedisplay device is extended.

In the example shown in FIG. 23, the pixels that emit the same color areprovided on the same row. Therefore, the number of the carrier injectionamount control signal lines Gn provided per row may be decreased. Forexample, in the display 112 d shown in FIG. 24, three carrier injectionamount control signal lines Gn_r, Gn_g and Gn_b, are provided per row.By contrast, in the display 112 c shown in FIG. 23, Gn_r is provided onthe n'th row, Gn+1_g is provided on the (n+1)'th row, and Gn_b isprovided on the (n+2)'th row. With such a structure, in the display 112c shown in FIG. 23, the amount of the carriers to be injected into thelight emitting layer of the EL element may be controlled for each colorof light, and thus the number of the carrier injection amount controlsignal lines Gn to be provided may be decreased.

2-2. Structure of the Display Device Including the EL Element

A structure of the display device including a pixel including the ELelement according to an embodiment of the present invention will bedescribed.

By way of the embodiment described below, the structures of the ELelement, the selection transistor, the driving transistor and thecapacitive element provided in the pixel will be described.

2-2-1. Structure 1 of the Display Device

FIG. 25 shows an example of layout of the pixel corresponding to theequivalent circuit shown in FIG. 11. FIG. 25 is a plan view of a pixel114 a. FIG. 26A shows a cross-sectional structure taken along line A1-A2in FIG. 25, and FIG. 26B shows a cross-sectional structure taken alongline B1-B2 in FIG. 25. FIG. 26A shows a cross-sectional structure of theEL element 102 and the driving transistor 106, and FIG. 26B shows across-sectional structure of the selection transistor 104 and thecapacitive element 108. The following description will be made withreference to FIG. 25, FIG. 26A and FIG. 26B. In FIG. 25, the stackstructure of the EL element 102 is omitted.

As shown in FIG. 25, the scanning signal line Sn, the data signal lineDm, the common line Cn, the common line Cm, and the carrier injectionamount control signal line Gn are provided in a region where the pixel114 a is provided. As shown in FIG. 26A and FIG. 26B, the scanningsignal line Sn, the common line Cn and the carrier injection amountcontrol signal line Gn are provided between the substrate 110 and thefirst insulating layer 126. The data signal line Dm and the common lineCm are provided between the first insulating layer 126 and the secondinsulating layer 142. The scanning signal line Sn, the common line Cnand the carrier injection amount control signal line Gn have the samelayer structure. Specifically, the scanning signal line Sn, the commonline Cn and the carrier injection amount control signal line Gn have astructure in which a transparent conductive layer 121 and a conductivelayer 123 formed of a low-resistance metal material are stacked on eachother. The data signal Dm and the common line Cm includes a metal layer127. The metal layer 127 may be held between a metal oxide conductivelayer 125 and an oxide semiconductor layer 131.

The carrier injection amount control signal line Gn has a structure inwhich a first transparent conductive layer 121 a formed of indium tinoxide (ITO) or the like and a first conductive layer 123 a formed of alow-resistance metal material such as aluminum (Al) or the like arestacked on each other. The carrier injection amount control signal lineGn is electrically connected with the first electrode 124 of the ELelement 102. Specifically, the first electrode 124 is formed in the samelayer as the first transparent conductive layer 121 a, and thus thecarrier injection amount control signal line Gn and the first electrode124 are electrically connected with each other. The carrier injectionamount control signal line Gn is formed of the stack structure of thefirst transparent conductive layer 121 a and the first conductive layer123 a, and therefore, is highly adhesive with an underlying layer and iselectrically connected with the first electrode 124 with no use of acontact hole. Hereinafter, the structure of each of components includedin the pixel 114 a will be described in detail.

2-2-1-1. Driving Transistor

The driving transistor 106 has a structure in which a first gateelectrode 152 a, the first insulating layer 126, a first oxidesemiconductor layer 131 a, the second insulating layer 142 and a secondgate electrode 154 a are stacked. The first gate electrode 152 a isprovided between the substrate 110 and the first insulating layer 126.The second gate electrode 154 a is provided as an upper layer to thesecond insulating layer 142 (provided on the side opposite to thesubstrate 110).

A first metal oxide conductive layer 125 a and a second metal oxideconductive layer 125 b are provided between the first insulating layer126 and the first oxide semiconductor layer 131 a. The first metal oxideconductive layer 125 a and the second metal oxide conductive layer 125 bare in contact with a surface of the first oxide semiconductor layer 131a on the side of the first insulating layer 126. The first metal oxideconductive layer 125 a and the second metal oxide conductive layer 125 bare provided to hold the first gate electrode 152 a and the second gateelectrode 154 a from both of two sides thereof as seen in a plan view.The first gate electrode 152 a has a layer structure same as that of thescanning signal line Sn. Namely, the first gate electrode 152 a has astructure in which a second transparent conductive layer 121 d and asecond conductive layer 123 d are stacked on each other. By contrast,the second gate electrode 154 a may have a single metal layer formed ofaluminum or the like.

The driving transistor 106 has a dual-gate structure in which the firstgate electrode 152 a and the second gate electrode 154 a overlap eachother with the first oxide semiconductor layer 131 a being providedtherebetween. The first insulating layer 126 between the first gateelectrode 152 a and the first oxide semiconductor layer 131 a, and thesecond insulating layer 142 between the second gate electrode 154 a andthe first oxide semiconductor layer 131 a, each act as a gate insulatingfilm. It is preferred that the first insulating layer 126 and the secondinsulating layer 142 are formed of an oxide insulating material such assilicon oxide or the like in order to suppress generation of a defectbased on oxygen deficiency of the first oxide semiconductor layer 131 a.The driving transistor 106 is not limited to having a dual-gatestructure, and may include only the first gate electrode 152 a or onlythe second gate electrode 154 a.

In the driving transistor 106, a region in which the first oxidesemiconductor layer 131 a is in contact with the first metal oxideconductive layer 125 a is a drain region, and a region where the firstoxide semiconductor layer 131 a is in contact with the second metaloxide conductive layer 125 b is a source region. Because of thisstructure, the first metal oxide conductive layer 125 a and a firstmetal layer 127 a substantially form a drain electrode 158 a, and thesecond metal oxide conductive layer 125 b and a second metal layer 127 bsubstantially form a source electrode 156 a.

The first metal layer 127 a is in contact with the first metal oxideconductive layer 125 a, whereas the second metal layer 127 b is incontact with the second metal oxide conductive layer 125 b. The firstmetal layer 127 a and the first metal layer 127 b are respectivelyprovided to decrease the sheet resistances of the first metal oxideconductive layer 125 a and the second metal oxide conductive layer 125b. The first metal oxide conductive layer 125 a and the first metallayer 127 a extend to the region of the EL element 102 and are providedto enclose the first opening 146 a and the second opening 146 b. Thesecond metal oxide conductive layer 125 b and the second metal layer 127b are electrically connected with the common line Cn via a contact holeformed in the first insulating layer 126. The common line Cn has astructure in which a third transparent conductive layer 131 c and athird conductive layer 123 c are stacked on each other.

The driving transistor 106 is covered with the third insulating layer144 and a passivation layer 143. The third insulating layer 144 isformed of an organic resin material such as, for example, acrylic resin,polyimide resin, epoxy resin, polyamide resin or the like. When acomposition containing a precursor of the organic resin material isapplied during the production of the EL display device 100, the thirdinsulating layer 144 has a surface thereof flattened by a levellingaction of a film of the applied composition. In another embodiment, thethird insulating layer 144 may be formed of an inorganic insulating filmof silicon oxide or the like by plasma CVD or the like and then have asurface thereof flattened by chemical mechanical polishing (CMP). It ispreferred that the passivation layer 143 is formed of silicon nitride.

2-2-1-2. Selection Transistor

The selection transistor 104 has a structure substantially the same asthat of the driving transistor 106. Namely, the selection transistor 104has a structure in which a first gate electrode 152 b, the firstinsulating layer 126, a second oxide semiconductor layer 131 b, thesecond insulating layer 142 and a second gate electrode 154 b arestacked. In the selection transistor 104, a channel is formed in aregion where the second oxide semiconductor layer 131 b overlaps thefirst gate electrode 152 b and the second gate electrode 154 b. A thirdmetal oxide conductive layer 125 c and a fourth metal oxide conductivelayer 125 d are provided between the first insulating layer 126 and thesecond oxide semiconductor layer 131 b. A region where the second oxidesemiconductor layer 131 b is in contact with the third metal oxideconductive layer 125 c is a source region, and a region where the secondoxide semiconductor layer 131 b is in contact with the fourth metaloxide conductive layer 125 d is a drain region.

Because of this structure, the third metal oxide conductive layer 125 cand a third metal layer 127 c substantially form a source electrode 156b, and the fourth metal oxide conductive layer 125 d and a fourth metallayer 127 d substantially form a drain electrode 158 b. The third metaloxide conductive layer 125 c and the fourth metal oxide conductive layer125 d are provided to hold the first gate electrode 152 a and the secondgate electrode 154 a from both of two sides thereof as seen in a planview.

The third metal oxide conductive layer 125 c and the third metal layer127 c are stacked on each other. The stack body of the third metal oxideconductive layer 125 c and the third metal layer 127 c forms the datasignal line Dm. The third metal layer 127 c substantially decreases thesheet resistance of the third metal oxide conductive layer 125 c. Thesecond oxide semiconductor layer 131 b extends to the region where thedata signal line Dm is located and is provided to cover the data signalline Dm. The data signal line Dm has a top surface and a side surfacethereof covered with the second oxide semiconductor layer 131 b, andtherefore, is not exposed to an oxidizing atmosphere or a reducingatmosphere during the production of the EL display device 100. For thisreason, the data signal line Dm may suppress surfaces of the third metaloxide conductive layer 125 c and the third metal layer 127 c from havingan increased resistance.

2-2-1-3. Capacitive Element

The capacitive element 108 is formed in a region where the common lineCn, the first insulating layer 126, the fourth metal oxide conductivelayer 125 d and the fourth metal layer 127 d are stacked. Namely, in theregion where the capacitive element 108 is formed, the stack of thecommon line Cn, the fourth metal oxide conductive layer 125 d and thefourth metal layer 127 d acts as a capacitive electrode.

The second oxide semiconductor layer 131 b and the second insulatinglayer 142 are provided as upper layers to the stack of the fourth metaloxide conductive layer 125 d and the fourth metal layer 127 d. The stackof the fourth metal oxide conductive layer 125 d and the fourth metallayer 127 d is electrically connected with the second gate electrode 154a via a contact hole running through the second insulating layer 142 andthe second oxide semiconductor layer 131 b. The second gate electrode154 a is electrically connected with the first gate electrode 152 a viaa contact hole running through the first insulating layer 126 and thesecond insulating layer 142.

2-2-1-4. EL element

The EL element 102 has a structure in which the first electrode 124, thefirst insulating layer 126, the electron transfer layer 130 (the firstelectron transfer layer 130 a and the second electron transfer layer 130b), the electron injection layer 132, the light emitting layer 134, thehole transfer layer 136, the hole injection layer 138 and the thirdelectrode 140 are stacked on the substrate 110 in this order from theside of the substrate 110. The electron transfer layer 130 and thesecond electrode 128 are electrically connected with each other. Thedetails of the structure of the EL element 102 are substantially thesame as those described above with reference to FIG. 1 and FIG. 2.

The EL element 102 is formed in a region where the first opening 146 arunning through the second insulating layer 142, and the second opening146 b running through the third insulating layer 140 and the passivationlayer 143, overlap each other. The first opening 146 a and the secondopening 146 b expose the first electron transfer layer 130 a. The secondelectron transfer layer 130 b, the electron injection layer 132, thelight emitting layer 134, the hole transfer layer 136, the holeinjection layer 138 and the third electrode (positive electrode) 140 arestacked on the first electron transfer layer 130 a. The second electrode128 is electrically connected with the drain of the driving transistor106, and is held between the first insulating layer 126 and the secondinsulating layer 142 to be insulated from the first electrode (carrierinjection amount control electrode) 124 and the third electrode(positive electrode) 140.

FIG. 26A shows a structure in which the first electron transfer layer130 a is continuous from the first oxide semiconductor layer 131 a ofthe driving transistor 106. The pixel 114 a is not limited to havingsuch a structure, and the first electron transfer layer 130 a and thefirst oxide semiconductor layer 131 a may be formed of separate layersnot continuous to each other.

As shown in FIG. 25, FIG. 26A and FIG. 26B, the first insulating layer126 and the second insulating layer 142 included in the EL element 102may be used as the gate insulating films of the driving transistor 106and the selection transistor 104. The conductive layer forming the firstelectrode 124 of the EL element 102 may be used to form the first gateelectrode 152 of the driving transistor 106 and the first gate electrode152 of the selection transistor 104. The conductive layer forming thesecond electrode 128 of the EL element 102 may be used to formelectrodes that are in contact with the sources and the drains of thedriving transistor 106 and the selection transistor 104. As describedabove, the EL element 102 according to an embodiment of the presentinvention may be formed by use of the same layers as the layers includedin the transistors included in the pixel. This may suppress theproduction cost from being increased.

2-2-2. Structure 2 of the Display Device

FIG. 27 shows another example of layout of the pixel corresponding tothe equivalent circuit shown in FIG. 11. FIG. 27 is a plan view of apixel 114 b. FIG. 28A shows a cross-sectional structure taken along lineA3-A4 in FIG. 27, and FIG. 28B shows a cross-sectional structure takenalong line B3-B4 in FIG. 27. FIG. 28A shows a cross-sectional structureof the EL element 102 and the driving transistor 106, and FIG. 28B showsa cross-sectional structure of the selection transistor 104 and thecapacitive element 108. The following description will be made withreference to FIG. 27, FIG. 28A and FIG. 28B. In FIG. 27, the stackstructure of the EL element 102 is omitted.

As shown in FIG. 27, the scanning signal line Sn, the data signal lineDm, the common line Cn, the common line Cm, and the carrier injectionamount control signal line Gn are provided in a region where the pixel114 b is provided. A layer in which the scanning signal line Sn, thecommon line Cn and the carrier injection amount control signal line Gnare formed is provided between the substrate 110 and the firstinsulating layer 126. The data signal line Dm and the common line Cm areprovided between the first insulating layer 126 and the secondinsulating layer 142. The common line Cn have a structure in which atransparent conductive layer 121 g and a conductive layer 123 g formedof a low-resistance metal material are stacked on each other. Thecarrier injection amount control signal line Gn have a structure inwhich a transparent conductive layer 121 f and a conductive layer 123 fformed of a low-resistance metal material are stacked on each other. Thedata signal line Dm (source electrode 156 b) includes a seventh metaloxide conductive layer 125 g and a seventh metal layer 127 g, and thedrain electrode 158 b includes an eighth metal oxide conductive layer125 h and an eighth metal layer 127 h. An interlayer insulating layer141 is provided to insulate first gate electrodes 152 c and 152 d fromthe source electrodes 156 a and 156 b and also from the drain electrodes158 a and 158 b.

In the driving transistor 106, a channel is formed in a firstsemiconductor layer 160 a. In the selection transistor 104, a channel isformed in a second semiconductor layer 160 b. The first semiconductorlayer 160 a and the second semiconductor layer 160 b are provided on thefirst insulating layer 126. A gate insulating layer 162 is provided onthe first semiconductor layer 160 a and the second semiconductor layer160 b. The first gate electrode 152 c includes a region overlapping thefirst semiconductor layer 160 a with the gate insulating layer 162 beingprovided between the region and the first semiconductor layer 160 a. Thesecond gate electrode 152 d includes a region overlapping the secondsemiconductor layer 160 b with the gate insulating layer 162 beingprovided between the region and the second semiconductor layer 160 b.The first semiconductor layer 160 a and the second semiconductor layer160 b are formed of polycrystalline silicon.

The EL element 102 has a structure in which the first electrode 124, thefirst insulating layer 126, the electron transfer layer 130 (the firstelectron transfer layer 130 a and the second electron transfer layer 130b), the electron injection layer 132, the light emitting layer 134, thehole transfer layer 136, the hole injection layer 138 and the thirdelectrode 140 are stacked in the first opening 146 a and the secondopening 146 b, and in which the second electrode 128 is connected withthe first electron transfer layer 130 a outside the first opening 146 aand the second opening 146 b. The second electrode 128 is provided toenclose the first opening 146 a and the second opening 146 b, and may beconnected with the drain electrode 158 a of the driving transistor 106or may be formed of an oxide semiconductor.

The driving transistor 106 and the selection transistor 104 are each ofan n-channel type. In the first semiconductor layer 160 a, a regionoverlapping the first gate electrode 152 c is a channel region, and aregion outer thereto is contaminated with an impurity element providingan n-type conductivity. Also in the second semiconductor layer 160 b, aregion overlapping the first gate electrode 152 d is a channel region,and a region outer thereto is contaminated with an impurity elementproviding an n-type conductivity. The common line Cn has a structure inwhich a transparent conductive layer 121 g and a conductive layer 123 gformed of a low-resistance metal material are stacked on each other. Thecapacitive element 108 is formed in a region where the n-type region ofthe second semiconductor layer 160 b and the common line Cn overlap eachother with the first insulating layer 126 being provided therebetween.

In the EL element 102, the first insulating layer 126 may be used as anunderlying insulating film for the first semiconductor layer 160 a andthe second semiconductor layer 160 b. Namely, the first insulating layer126 provides a structure in which neither the first semiconductor layer160 a nor the second semiconductor layer 160 b is in direct contact withthe substrate 110. Thus, the first insulating layer 126 prevents thefirst semiconductor layer 160 a and the second semiconductor layer 160 bfrom being contaminated by the substrate 110 to improve the reliability.The second insulating layer 142 insulating the second electrode 128 andthe third electrode 140 from each other is provided as an upper layer tothe source electrodes 156 a and 156 b and the drain electrodes 158 a and158 b. The third insulating layer 144 is provided as an upper layer tothe second insulating layer 142. The EL elements 102 can be provided onthe same substrate with no influence on the structure of the drivingtransistor 106 and the selection transistor 104.

As shown in FIG. 27, FIG. 28A and FIG. 28B, the driving transistor 106and the selection transistor 104 may be formed of a siliconsemiconductor, so that the display device is formed in substantially thesame manner as described above. In the case where the firstsemiconductor layer 160 a and the second semiconductor layer 160 b areformed of a polycrystalline silicon semiconductor, a higher mobility isrealized than in the case where the first semiconductor layer 160 a andthe second semiconductor layer 160 b are formed of an oxidesemiconductor. Therefore, the display device may be driven even when theframe frequency is higher.

2-2-3. Structure 3 of the Display Device

FIG. 24 shows a pixel circuit in which the first data signal line D1 mand the second data signal line D2 m are provided on one column. FIG. 29shows an example of layout corresponding to the pixel circuit shown inFIG. 24. FIG. 30A shows a cross-sectional structure taken along lineC1-C2 in FIG. 29, and FIG. 30B shows a cross-sectional structure takenalong line C3-C4 in FIG. 29.

FIG. 29 shows a partial structure of the first pixel 114(n) and thesecond pixel 114(n+1) located adjacent to each other in the columndirection. The details of the structure of the first pixel 114(n) andthe second pixel 114(n+1) are substantially the same as those in FIG.25. In the first pixel 114(n) belonging to the n'th row, the selectiontransistor 104(n) is electrically connected to the first data signalline D1 m. In the second pixel 114(n+1) belonging to the (n+1)'th row,the selection transistor 104(n+1) is electrically connected to thesecond data signal line D2 m. The first data signal line D1 m and thesecond data signal line D2 m are located to overlap each other as seenin a plan view. FIG. 29 shows an embodiment in which the first datasignal line D1 m is provided in an upper layer and the second datasignal line D2 m is provided in a lower layer.

The selection transistor 104(n) of the first pixel 114(n) and theselection transistor 104(n+1) of the second pixel 114(n+1) have the samelayer structure as each other (are provided in the same layer). A sourceelectrode 156 b(n) of the first pixel 114(n) is electrically connectedwith the first data signal line D1 m. The source electrode 156 b(n) andthe first data signal line D1 m are provided in the same layer. In aregion where the selection transistor 104(n+1) of the second pixel114(n+1) is electrically connected with the second data signal line D2m, the first data signal line D1 m is provided to be curved so as not tointerfere with a source electrode 156 b(n+1) of the selection transistor104(n+1).

FIG. 30A shows a cross-sectional structure of a connection portion ofthe selection transistor 104(n) of the first pixel 114(n) and the firstdata signal line D1 m. FIG. 30B shows a cross-sectional structure of aconnection portion of the selection transistor 104(n+1) of the secondpixel 114(n+1) and the second data signal line D2 m. Like in FIG. 26,the first data signal line D1 m is provided between the first insulatinglayer 126 and the second insulating layer 142. The second data signalline D2 m is provided as a lower layer to the first insulating layer126. The second data signal line D2 m is provided to be embedded in aninsulating layer 164 so as not to interfere with the scanning signalline Sn.

As shown in FIG. 30A, the first data signal line D1 m and the seconddata signal line D2 m are located to overlap each other. The first datasignal line D1 m and the second data signal line D2 m are supplied withdata signals of different voltage levels. Therefore, delay of thesignals caused by the parasitic capacitance needs to be considered.However, since the first insulating layer 126 and the second insulatinglayer 142 are provided between the first data signal line D1 m and thesecond data signal line D2 m, the distance between the lines is extendedand the influence of the parasitic capacitance is decreased.

As shown in FIG. 29 and FIG. 30B, the second data signal line D2 m iselectrically connected with the source electrode 156 b(n+1) of theselection transistor 104(n+1) of the second pixel 114(n+1) via a contacthole 165 formed in the first insulating layer 126 and the insulatinglayer 164. The source electrode 156 b(n+1), which has a stack structureof a third metal oxide conductive layer 125 c(n+1) and a third metallayer 127 c(n+1), has a sufficient thickness, and therefore, is notdisconnected in the contact hole 165. The second data signal line D2 mis formed of a metal material. For example, the second data signal lineD2 m has a structure in which a second metal layer 166 b formed of ahighly conductive material such as aluminum (Al), copper (Cu) or thelike is provided between a first metal layer 166 a and a third metallayer 166 c each formed of a high melting point metal material such asmolybdenum (Mo), titanium (Ti) or the like. The second data signal lineD2 m is formed of a highly conductive metal material such as aluminum(Al), copper (Cu) or the like, and therefore, may decrease the lineresistance thereof.

FIG. 31A shows a structure including, in addition to the componentsshown in FIG. 30A, a color filter layer 168 g provided as a lower layerto the insulating layer 164, namely, between the insulating layer 164and the substrate 110. FIG. 31B also shows a structure including a colorfilter layer 168 r in addition to the components shown in FIG. 30B. Theinsulating layer 164 is used as a flattening film that prevents the stepprovided by the second data signal line D2 m from being reflected in thelayers on the insulating layer 164. Therefore, the color filter layers168 g and 168 r are embedded in the insulating layer 164, like the colorfilter layer 168 r. The insulating layer 164 may have a thickness of 3to 5 μm, so as to embed the second data signal line D2 m and the colorfilter layers 168 r and 168 g. The insulating layer 164 flattens topsurfaces of the second data signal line D2 m and the color filter layers168 r and 168 g, and therefore, the EL element and the transistors maybe formed thereon. The structures shown in FIG. 31A and FIG. 31B areusable in the case where the EL element included in the pixel is of thebottom-emission type as shown in FIG. 1 and the light emitting layers134 of the EL elements 102 of the display 112 all emit blue light.

FIG. 31A and FIG. 31B each show a structure in which the red colorfilter layer 168 r and the green color filter layer 168 g are separatedfrom each other by the second data signal line D2 m. As shown in FIG.29, the second data signal line D2 m is provided on each column. Thesecond data signal line D2 m is used as a line transmitting a signal andis also usable as a black matrix (light blocking film) demarcating thecolor filter layers on a column-by-column basis. In the case where theEL display device of the bottom-emission type adopts a so-calledarray-on-color filter structure, the second data signal line D2 m may beused as a black matrix, so that the structure and the production processof the EL display device are simplified. FIG. 31A and FIG. 31B show onlythe red color filter layer 168 r and the green color filter layer 168 g.In actuality, color filter layers corresponding to R (red), green (G)and blue (B) are provided, and a color filter layer corresponding toanother color (e.g., yellow (Y)) may be optionally provided.

FIG. 31A and FIG. 31B each show a case where the color filter layers 168r and 168 g are formed by use of quantum dots (QD). Specifically, thered color filter 168 r includes a red-colored layer 168 r_1 and ared-converted quantum dot-containing layer 168 r_2 stacked from the sideof the substrate 110. The green color filter 168 g includes agreen-colored layer 168 g_1 and a green-converted quantum dot-containinglayer 168 g_2 stacked from the side of the substrate 110. In thismanner, the colored layer and the color-converted layer containing thequantum dots may be combined, so that the color purity of each of thered pixel and the green pixel is improved. Although not shown, ablue-colored layer may be provided in the region corresponding to theblue pixel, and a blue color scattering layer may be stacked thereon.

As shown in FIG. 29, FIG. 30A and FIG. 30B, the first data signal lineD1 m and the second data signal line D2 m may be provided on the samecolumn, so that the signal is written to two rows at the same time. Inthis case, the first data signal line D1 m and the second data signalline D2 m may be provided to overlap each other with the insulatinglayer being provided therebetween. With such a structure, even if thepixel pitch is decreased, the numerical aperture may be suppressed frombeing decreased to realize a high definition display panel. As shown inFIG. 31A and FIG. 31B, the color filter layers 168 r and 168 g may beprovided, so that the color purity of the display device of thebottom-emission type is improved.

As described above, in the EL display device according to an embodimentof the present invention, an electrode that controls the carrierinjection amount is provided in the EL element. This may suppress thedeterioration in the luminance and improve the reliability of the ELdisplay device. In order to apply a signal to the carrier injectionamount control electrode, a dedicated line (carrier injection amountcontrol signal line) needs to be provided. Such a carrier injectionamount control signal line may be shared by two adjacent rows, so thatthe number of the lines is suppressed from being increased. The signalapplied to the carrier injection amount control electrode controls thelight emission period of the EL element. With such a structure, even inthe case where, for example, the frame frequency is 60 Hz, the movingimage characteristic are improved.

1. A driving method of a display device, the display device comprisingan EL element, the method comprising: applying a first voltage to acarrier injection amount control electrode to make the EL elementnon-luminescent in a first horizontal period, wherein the carrierinjection amount control electrode arranged to overlap an overlappingregion of an electron transfer layer, a light emitting layer, an anodewith the insulating layer interposed therebetween; applying a forwardvoltage to the EL element in the first horizontal period; and applying asecond voltage higher than the first voltage to the carrier injectionamount control electrode to emit light from the EL element in a lightemission period after the first horizontal period.
 2. The driving methodaccording to claim 1, wherein the second voltage is a constant positivevoltage.
 3. The driving method according to claim 1, wherein the secondvoltage is fluctuating voltage that changes between a positive voltageof a first level and a positive voltage of a second level higher thanthe first level.
 4. The driving method according to claim 1, wherein thesecond voltage is stepwise wave that increase or decrease.
 5. Thedriving method according to claim 1, wherein the second voltage is apositive voltage that decreases from a first level V1 to a second levelV2 and then decreases from the second level V2 to a third level V3 aftera certain period of time.
 6. The driving method according to claim 1,wherein the second voltage is a positive voltage that changes in asinusoidal shape.
 7. A driving method of a display device, the displaydevice comprising an EL element and a driving transistor electricallyconnected to the EL element, the method comprising: applying a firstvoltage to a carrier injection amount control electrode to make the ELelement non-luminescent in a first horizontal period, wherein thecarrier injection amount control electrode arranged to overlap anoverlapping region of an electron transfer layer, a light emittinglayer, an anode with the insulating layer interposed therebetween;applying a data voltage to the driving transistor to forward bias the ELelement in the first horizontal period; and applying a second voltagehigher than the first voltage to the carrier injection amount controlelectrode to emit light from the EL element in a light emission periodafter the first horizontal period.
 8. The driving method according toclaim 7, wherein the second voltage is a constant positive voltage. 9.The driving method according to claim 7, wherein the second voltage isfluctuating voltage that changes between a positive voltage of a firstlevel and a positive voltage of a second level higher than the firstlevel.
 10. The driving method according to claim 7, wherein the secondvoltage is stepwise wave that increase or decrease.
 11. The drivingmethod according to claim 7, wherein the second voltage is a positivevoltage that decreases from a first level V1 to a second level V2 andthen decreases from the second level V2 to a third level V3 after acertain period of time.
 12. The driving method according to claim 7,wherein the second voltage is a positive voltage that changes in asinusoidal shape.